RFC: Simpelify packaging by splitting out collection of git version info

This is intended to be part of the solution to https://github.com/JuliaLang/julia/issues/5068. I tried to look at the submodules also, but I was confused, and hope someone else will tackle that problem.

It would be nice to get some feedback on the changes. I have only tried it on Ubuntu, because I tried to dual boot my MacBook Pro, but now I can't boot into OSX.

* Renamed BUILD_INFO to GIT_VERSION_INFO (I'm not sure about this one, but it makes sense)
* Put the git parts of build_h.jl into version_git.jl
* Made sure that an existing version_git.jl file does not get overwritten if
  if no git information is availible.