build: default selinuxfs mount point to /sys/fs/selinux

Currently if you build on a machine that does not support SELinux we end up
with the default mount point being /selinux, since this is moved to
/sys/fs/selinux, we should start defaulting there.

I believe this is causing a bug in libvirt-lxc when /selinux does not exists,
even though /sys/fs/selinux exists.
(cherry picked from commit aa696e18)

configure.ac

@@ -1455,9 +1455,8 @@ fi
 if test "$with_selinux" = "yes"; then
   AC_MSG_CHECKING([SELinux mount point])
   if test "$with_selinux_mount" = "check" || test -z "$with_selinux_mount"; then
-    if test -d /sys/fs/selinux ; then
-      SELINUX_MOUNT=/sys/fs/selinux
-    else
+    SELINUX_MOUNT=/sys/fs/selinux
+    if ! test -d ${SELINUX_MOUNT} && test -d /selinux ; then
       SELINUX_MOUNT=/selinux
     fi
   else