fix: add -D_GNU_SOURCE to CFLAGS for strcasestr visibility on Linux

Author: trollkotze

Makefile.cbm

@@ -45,7 +45,7 @@ GCC_ONLY_FLAGS := -Wno-format-truncation -Wno-unused-result \
                   -Wno-stringop-truncation -Wno-alloc-size-larger-than
 endif
 
-CFLAGS_COMMON = -std=c11 -D_DEFAULT_SOURCE -Wall -Wextra -Werror \
+CFLAGS_COMMON = -std=c11 -D_DEFAULT_SOURCE -D_GNU_SOURCE -Wall -Wextra -Werror \
                 -Wno-unused-parameter -Wno-sign-compare \
                 $(GCC_ONLY_FLAGS) \
                 -Isrc -Ivendored -Ivendored/sqlite3 \