selinux: fix Makefile for versions of make < v4.3

As noted in the comments of this commit, the current SELinux Makefile
requires features found in make v4.3 or later, which is problematic
as the Linux Kernel currently only requires make v3.82.  This patch
fixes the SELinux Makefile so that it works properly on these older
versions of make, and adds a couple of comments to the Makefile about
how it can be improved once make v4.3 is required by the kernel.

Fixes: 6f933aa7 ("selinux: more Makefile tweaks")
Signed-off-by: Paul Moore <paul@paul-moore.com>