fix warnings when compiling with gcc8

[nnathan]

E.g. when compiling with gcc8 with warnings as errors you'll get
the following:

mode_s.c: In function 'nav_modes_to_string':
mode_s.c:1626:9: error: 'strncat' specified bound 10 equals source length [-Werror=stringop-overflow=]
strncat(buf, "autopilot ", 10);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This error is indicating that you should be using strcat instead of strncat
since the length of source is same as the bound.

[Mictronics]

I cannot reproduce this error on Ubuntu 18.04.
My compiler call:
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -Werror=stringop-overflow -c mode_s.c -o mode_s.o

[gregory-sh]

building on the new raspbian buster with gcc (Raspbian 8.3.0-6+rpi1) 8.3.0
and the command make BLADERF=no
i am also seeing this issue,

cc -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/local/include/ -I/usr/include/libusb-1.0 -I/usr/local/include -c dump1090.c -o dump1090.o
cc -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/local/include/ -I/usr/include/libusb-1.0 -I/usr/local/include -c anet.c -o anet.o
cc -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/local/include/ -I/usr/include/libusb-1.0 -I/usr/local/include -c interactive.c -o interactive.o
cc -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/local/include/ -I/usr/include/libusb-1.0 -I/usr/local/include -c mode_ac.c -o mode_ac.o
cc -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/local/include/ -I/usr/include/libusb-1.0 -I/usr/local/include -c mode_s.c -o mode_s.o
mode_s.c: In function ‘nav_modes_to_string’:
mode_s.c:1626:9: error: ‘strncat’ specified bound 10 equals source length [-Werror=stringop-overflow=]
         strncat(buf, "autopilot ", 10);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mode_s.c:1630:9: error: ‘strncat’ specified bound 8 equals source length [-Werror=stringop-overflow=]
         strncat(buf, "althold ", 8);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
mode_s.c:1632:9: error: ‘strncat’ specified bound 9 equals source length [-Werror=stringop-overflow=]
         strncat(buf, "approach ", 9);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
mode_s.c:1634:9: error: ‘strncat’ specified bound 5 equals source length [-Werror=stringop-overflow=]
         strncat(buf, "lnav ", 5);
         ^~~~~~~~~~~~~~~~~~~~~~~~
mode_s.c:1636:9: error: ‘strncat’ specified bound 5 equals source length [-Werror=stringop-overflow=]
         strncat(buf, "tcas ", 5);
         ^~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [Makefile:60: mode_s.o] Error 1

[Mictronics]

Having gcc (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0 and compiling with full support:

/usr/bin/make -f Makefile all HTMLPATH=/usr/share/dump1090-fa/html BLADERF=yes RTLSDR=yes AGGRESSIVE=no PLUTOSDR=yes HAVE_BIASTEE=no CC=gcc-8
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c dump1090.c -o dump1090.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c anet.c -o anet.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c interactive.c -o interactive.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c mode_ac.c -o mode_ac.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c mode_s.c -o mode_s.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c comm_b.c -o comm_b.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c net_io.c -o net_io.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c crc.c -o crc.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c demod_2400.c -o demod_2400.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c stats.c -o stats.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c cpr.c -o cpr.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c icao_filter.c -o icao_filter.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c track.c -o track.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c util.c -o util.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c convert.c -o convert.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c sdr_ifile.c -o sdr_ifile.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c sdr_beast.c -o sdr_beast.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c sdr.c -o sdr.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c ais_charset.c -o ais_charset.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c sdr_rtlsdr.c -o sdr_rtlsdr.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c sdr_bladerf.c -o sdr_bladerf.o
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c sdr_plutosdr.c -o sdr_plutosdr.o
gcc-8 -g -o dump1090 dump1090.o anet.o interactive.o mode_ac.o mode_s.o comm_b.o net_io.o crc.o demod_2400.o stats.o cpr.o icao_filter.o track.o util.o convert.o sdr_ifile.o sdr_beast.o sdr.o ais_charset.o sdr_rtlsdr.o sdr_bladerf.o sdr_plutosdr.o  -lpthread -lm -lrt -lrtlsdr -lusb-1.0 -L/usr/lib/x86_64-linux-gnu/ -lbladeRF -L/usr/local/lib -liio -lad9361 -lncurses
gcc-8 -DMODES_DUMP1090_VERSION=\"'Mictronics base v3.7.1'\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\" -D_GNU_SOURCE -DENABLE_RTLSDR -DENABLE_BLADERF -DENABLE_PLUTOSDR -std=c11 -O2 -g -W -D_DEFAULT_SOURCE -Wall -Werror -I/usr/include/ -I/usr/include/ -I/usr/local/include -c view1090.c -o view1090.o
gcc-8 -g -o view1090 view1090.o anet.o interactive.o mode_ac.o mode_s.o comm_b.o net_io.o crc.o stats.o cpr.o icao_filter.o track.o util.o ais_charset.o  -lpthread -lm -lrt -lncurses

BUILD SUCCESSFUL (total time: 4s)

[Mictronics]

Don't know why there is a difference between Ubuntu and Raspian buster but let's get this fixed.
Thanks for contribution.