Support building of system image in binary builds

I'm going to wait for travis to turn green before I merge
I guess it could on Windows and OS X. But it looks like it's a rabbit hole: on OS X, you should also decompose the Unicode path. So the most reliable solution is to try to open both files and see whether they point to the same inode, which is what os.path.samefile does in Python. It would certainly be a useful addition to Julia.

See the recent discussion at https://lwn.net/Articles/626898/