makefiles: stop failing when config.mk does not exist

This allows running `make clean` and `make distclean` (and possibly
others) without having to run ./configure beforehand.

Note that some packaging-related targets still depend on the existence
of generated files.  For example:

* dist: config.mk
* deb: config.sh

Commands used to search and replace:

    $ git grep -Elz 'include *([^ ]*/)?config.mk' | xargs -0 -I '{}' \
      sh -c "printf '%s\n' \
      \"\$(sed -E 's|^include *(([^ ]*/)?config.mk)|-include \1|' '{}')\" >'{}'"

Relates to netblue30#5140.

Makefile

@@ -1,4 +1,4 @@
-include config.mk
+-include config.mk
 
 ifneq ($(HAVE_MAN),no)
 MAN_TARGET = man

src/bash_completion/Makefile

@@ -2,7 +2,7 @@
 all: firejail.bash_completion
 
 ROOT = ../..
-include $(ROOT)/config.mk
+-include $(ROOT)/config.mk
 
 firejail.bash_completion: firejail.bash_completion.in
  gawk -f ../man/preproc.awk -- $(MANFLAGS) < $< > $@.tmp

src/common.mk

@@ -2,7 +2,7 @@
 #
 # Note: "ROOT" must be defined before including this file.
 
-include $(ROOT)/config.mk
+-include $(ROOT)/config.mk
 
 H_FILE_LIST = $(sort $(wildcard *.h))
 C_FILE_LIST = $(sort $(wildcard *.c))

src/libpostexecseccomp/Makefile

@@ -1,5 +1,5 @@
 ROOT = ../..
-include $(ROOT)/config.mk
+-include $(ROOT)/config.mk
 
 H_FILE_LIST = $(sort $(wildcard *.h))
 C_FILE_LIST = $(sort $(wildcard *.c))

src/libtrace/Makefile

@@ -1,5 +1,5 @@
 ROOT = ../..
-include $(ROOT)/config.mk
+-include $(ROOT)/config.mk
 
 H_FILE_LIST = $(sort $(wildcard *.h))
 C_FILE_LIST = $(sort $(wildcard *.c))

src/libtracelog/Makefile

@@ -1,5 +1,5 @@
 ROOT = ../..
-include $(ROOT)/config.mk
+-include $(ROOT)/config.mk
 
 H_FILE_LIST = $(sort $(wildcard *.h))
 C_FILE_LIST = $(sort $(wildcard *.c))

src/man/Makefile

@@ -2,7 +2,7 @@
 all: firecfg.man firejail.man firejail-login.man firejail-users.man firejail-profile.man firemon.man jailcheck.man
 
 ROOT = ../..
-include $(ROOT)/config.mk
+-include $(ROOT)/config.mk
 
 %.man: %.txt
  gawk -f ./preproc.awk -- $(MANFLAGS) < $< > $@

src/zsh_completion/Makefile

@@ -2,7 +2,7 @@
 all: _firejail
 
 ROOT = ../..
-include $(ROOT)/config.mk
+-include $(ROOT)/config.mk
 
 _firejail: _firejail.in
  gawk -f ../man/preproc.awk -- $(MANFLAGS) < $< > $@.tmp