Modernize usage of autoconf macros

[robert-scheck]

When running autoreconf --install (also art of ./bootstrap), some warnings related to autoconf are raised:

$ autoreconf --install
aclocal: warning: couldn't open directory 'm4': No such file or directory
libtoolize: putting auxiliary files in '.'.
libtoolize: copying file './ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'.
libtoolize: copying file 'm4/libtool.m4'
libtoolize: copying file 'm4/ltoptions.m4'
libtoolize: copying file 'm4/ltsugar.m4'
libtoolize: copying file 'm4/ltversion.m4'
libtoolize: copying file 'm4/lt~obsolete.m4'
configure.ac:59: warning: The macro 'AC_PROG_CC_STDC' is obsolete.
configure.ac:59: You should run autoupdate.
./lib/autoconf/c.m4:1669: AC_PROG_CC_STDC is expanded from...
configure.ac:59: the top level
configure.ac:61: warning: The macro 'AC_PROG_LIBTOOL' is obsolete.
configure.ac:61: You should run autoupdate.
m4/libtool.m4:100: AC_PROG_LIBTOOL is expanded from...
configure.ac:61: the top level
configure.ac:22: installing './compile'
configure.ac:18: installing './config.guess'
configure.ac:18: installing './config.sub'
configure.ac:19: installing './install-sh'
configure.ac:19: installing './missing'
Makefile.am: installing './depcomp'
$ 

• AC_PROG_CC_STDC is meanwhile done by AC_PROG_CC, see: https://www.gnu.org/software/autoconf/manual/autoconf-2.70/html_node/Obsolete-Macros.html
• AC_PROG_LIBTOOL can be replaced by LT_INIT since libtool v2, dates back to ~ 2008, see also: https://autotools.info/forwardporting/libtool.html
• Finally, autoconf loves square brackets ;-)

Running autoreconf --install with the suggested changes applied looks finally like this:

$ autoreconf --install
aclocal: warning: couldn't open directory 'm4': No such file or directory
libtoolize: putting auxiliary files in '.'.
libtoolize: copying file './ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'.
libtoolize: copying file 'm4/libtool.m4'
libtoolize: copying file 'm4/ltoptions.m4'
libtoolize: copying file 'm4/ltsugar.m4'
libtoolize: copying file 'm4/ltversion.m4'
libtoolize: copying file 'm4/lt~obsolete.m4'
configure.ac:22: installing './compile'
configure.ac:18: installing './config.guess'
configure.ac:18: installing './config.sub'
configure.ac:19: installing './install-sh'
configure.ac:19: installing './missing'
Makefile.am: installing './depcomp'
$ 

To avoid breaking builds of bgpq4 in maybe non-edge Linux distributions, I've also tested this change with both:

• autoconf 2.72 (Fedora Rawhide, currently 41)
• autoconf 2.69 (RHEL/CentOS 7)

Note that autoconf 2.69 was released in April 2012. When looking to Repology, autoconf >= 2.69 seems to be in all common non-EOL distributions out there.