Move OpamGitVersion to the end of compilation

Rather than the first file, to avoid full recompilation on every
commit (or rather, plain wrong `opam --git-version`)

Of course, it's no longer available in the lib, only in the exe.

You'll have to re-run ./configure after this patch
Merge pull request #1917 from OCamlPro/github-version-end

Move OpamGitVersion to the end of compilation