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

[staticfloat]

Ref #5181. I tested this and it seems to work on OSX. (Via make source-dist in one directory, then unpacking the resultant tarball to another directory and building the whole tree. Unfortunately, this took over 45 minutes, so if you decide to test this, gentle reader, please ensure that you've got something else to occupy your time before rashly typing make)

• 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 RFC: Simpelify packaging by splitting out collection of git version info #5095. 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

[ivarne]

I think I have an improved version where I do not hard code that much knowledge about the structure of the julia repository because I use git ls-files to get a list of all the files in the repository. I just needed a little push and some assurance that I did not forget any dependencies. Expect a new PR soon.