diff --git a/acinclude.m4 b/acinclude.m4
index ac29c2bb..13fd52ca 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -12,7 +12,7 @@ AC_DEFUN([AC_PROG_CC_PIE], [
 
 AC_DEFUN([COMPILER_FLAGS], [
 	if (test "${CFLAGS}" = ""); then
-		CFLAGS="-Wall -O2 -D_FORTIFY_SOURCE=2"
+		CFLAGS="-Wall -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
 	fi
 	if (test "$USE_MAINTAINER_MODE" = "yes"); then
 		CFLAGS="$CFLAGS -Werror -Wextra"