build: Avoid redefining _FORTIFY_SOURCE

Gentoo (and others) define _FORTIFY_SOURCE=2 by default.  Since we
always define it as well, this results in redefinition warnings &
configure failure.

acinclude.m4

@@ -12,7 +12,8 @@ AC_DEFUN([AC_PROG_CC_PIE], [
 
 AC_DEFUN([COMPILER_FLAGS], [
 	if (test "${CFLAGS}" = ""); then
-		CFLAGS="-Wall -O2 -D_FORTIFY_SOURCE=2 -fsigned-char"
+		CFLAGS="-Wall -O2 -fsigned-char "
+		CFLAGS+=" -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
 	fi
 	if (test "$USE_MAINTAINER_MODE" = "yes"); then
 		CFLAGS+=" -Werror -Wextra"