Add source-dist target to Makefile for creating gitless source tarball

Ref #5181 

I tested this and it seems to work on OSX.

* This currently just downloads all deps via `make -C deps getall`.  This gets platform-specific deps such as `patchelf` as well, so I hope that this source tarball is cross-platform.

* This includes the `version_git.jl.phony` stuff from #5059.  Once this is merged, I hope that can be merged quickly as well, it directly addresses the most common Ubuntu build failure quite nicely.

@ivarne @nalimilan