Fix make error for Fedora 42

[michaelortmann]

Found by: robert-scheck
Patch by: michaelortmann
Fixes: #1740

One-line summary:
Add -std=gnu99, if available, to CFLAGS

Additional description (if needed):
GCC 15 changes the default language version for C compilation from -std=gnu17 to -std=gnu23. See https://gcc.gnu.org/gcc-15/changes.html

Test cases demonstrating functionality (if applicable):
Test with gcc-15-20250112
Before:

/home/michael/opt/gcc-15-20250112/bin/gcc -g -O2 -pipe -Wall -I.. -I..  -DHAVE_CONFIG_H -I/usr/include  -c botcmd.c
botcmd.c: In function ‘bot_filereq’:
botcmd.c:1155:9: error: too many arguments to function ‘f’; expected 0, have 3
 1155 |         f(idx, from, path);
      |         ^ ~~~
make[1]: *** [Makefile:86: botcmd.o] Error 1
make[1]: Leaving directory '/home/michael/projects/eggdrop/src'
make: *** [Makefile:243: eggdrop] Error 2

After:
/home/michael/opt/gcc-15-20250112/bin/gcc -std=gnu99 -g -O2 -pipe -Wall -I.. -I.. -DHAVE_CONFIG_H -I/usr/include -c botcmd.c