gentoo bug #498794
--- src/aclocal.m4	2014-01-16 00:44:15.000000000 +0000
+++ src/aclocal.m4	2014-03-26 10:20:54.000000000 +0000
@@ -501,6 +501,7 @@
   fi
   if test "x$krb5_ac_warn_cflags_set" = xset ; then
     AC_MSG_NOTICE(not adding extra gcc warning flags because WARN_CFLAGS was set)
+    WARN_CFLAGS=""
   else
     AC_MSG_NOTICE(adding extra warning flags for gcc)
     WARN_CFLAGS="$WARN_CFLAGS $extra_gcc_warn_opts -Wmissing-prototypes"