Add -std=gnu99, if available, to CFLAGS

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

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

Author: michaelortmann

configure.ac

@@ -54,6 +54,7 @@ AC_USE_SYSTEM_EXTENSIONS
 EGG_CHECK_ICC
 EGG_CHECK_CCPIPE
 EGG_CHECK_CCWALL
+AX_CHECK_COMPILE_FLAG([-std=gnu99], [CFLAGS="-std=gnu99 $CFLAGS"])
 
 
 # Check for executable extension.