Better self-package def description

I use this a lot to test the new pinnings
Fixed Makefile for running outside of git

Also, not refreshing OpamGitVersion unless you 'make clean', it may get a bit
out of date but, otherwise, we'd be rebuilding everything on every commit.
Or we could make only OpamArg depend on it rather than the base lib...
Fixed 'opam pin add .'
Merge pull request #1514 from OCamlPro/pin-fix

small pin & build fixes