redhat/Makefile: Add dist-get-buildreqs target

The dist-buildreq-check target is useful in a few ways but results in
unnecessary errors in many cases. Instead of stopping the build, add a
new target 'dist-get-buildreqs' that can be called to get a list of
uninstalled build requirements.

Signed-off-by: Prarit Bhargava prarit@redhat.com