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

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 #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