make.bash: stricter selinux test

SELinux can be installed but not turned on.
Don't complain unless it is actually turned on.

https://bugzilla.redhat.com/show_bug.cgi?id=652987#c20

R=agl
CC=golang-dev
https://golang.org/cl/3979044

src/make.bash

@@ -36,7 +36,7 @@ rm -f "$GOBIN"/gomake
 ) >"$GOBIN"/gomake
 chmod +x "$GOBIN"/gomake
 
-if [ -d /selinux -a -f /selinux/booleans/allow_execstack ] ; then
+if [ -d /selinux -a -f /selinux/booleans/allow_execstack -a -x /usr/sbin/selinuxenabled ] && /usr/sbin/selinuxenabled; then
 	if ! cat /selinux/booleans/allow_execstack | grep -c '^1 1$' >> /dev/null ; then
 		echo "WARNING: the default SELinux policy on, at least, Fedora 12 breaks "
 		echo "Go. You can enable the features that Go needs via the following "