Add make dist and make srpm targets

Add make dist and make srpm targets

Add targets to generate distribution tarballs, and srpms, from a tree.
These will generate packages for whatever the current HEAD of the tree
is - if the HEAD is a release tag, then the packages will be named for
that release, if the HEAD is between releases, then git describe will
be used to create an appropriate version identifier.

The tarballs are generated from the current git repository contents,
anything not checked in will not be included.

Reviewed-on: http://gerrit.openafs.org/4984
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit f775c0fb6f9191dcf5d226bfddbf82f52cd1116c)

Change-Id: I03b4f02d979bbc851e043e5979a54d00f9c52439
Reviewed-on: http://gerrit.openafs.org/5009
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>