auto merge of #12274 : brson/rust/mkfiles, r=alexcrichton

I've been working on binary installers and ended up taking this detour, which does a few things:

* It expands the documentation on the build system with new comments in Makefile.in
* It displays some of that documentation via `make help`
* Removes some unused and broken snapshot code
* Adds `NO_MKFILE_DEPS` to convenience makefile hacking
* Moves almost all of Makefile.in to files in `mk/`

The documentation provided by `make help` and its implementation are somewhat quirky.

Author: bors