Fix typo in acinclude.m4 fs_old_*flags

Introduced in commit d2669b47.

Signed-off-by: Dan McGee <dan@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>

m4/acinclude.m4

@@ -58,11 +58,11 @@ AC_DEFUN([GCC_FORTIFY_SOURCE_CC],[
       }
     ], [
       AC_MSG_RESULT(yes)
-      CFLAGS="$df_old_cflags"
+      CFLAGS="$fs_old_cflags"
     ], [
       AC_MSG_RESULT(no)
-      CPPFLAGS="$df_old_cppflags"
-      CFLAGS="$df_old_cflags"
+      CPPFLAGS="$fs_old_cppflags"
+      CFLAGS="$fs_old_cflags"
     ])
   fi
 ])