bdr: add a git-dist target to the Makefile

bdr: add a git-dist target to the Makefile

You can now prepare dist tarballs of the BDR extension. They'll contain the BDR
version in the filename and will have the git revision in a .distgitrev file in
the tarball. There's also a .distgittag file that contains the tag if there's
one, or 'undefined' if there isn't.

Document making dist tarballs in the README.developers too.