build: use -Wformat-security

m4/compiler_warnings.m4

@@ -11,7 +11,7 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		      -Wdeclaration-after-statement -Wstrict-prototypes \
 		      -Wno-unused-parameter -Wno-sign-compare \
 		      -fno-strict-aliasing -Wno-deprecated-declarations \
-		      -Wno-unused-but-set-variable; do
+		      -Wno-unused-but-set-variable -Wformat-security; do
 		SAVE_CFLAGS="$CFLAGS"
 		CFLAGS="$CFLAGS $option"
 		AC_MSG_CHECKING([whether gcc understands $option])