Build some dependencies with rustpkg

Also take a rust-layers bugfix.

In collaboration with Jack Moffitt.
Pass Mac OS version cfg flags to rust-opengles build

This was in the rust-opengles Makefile.in pre-rustpkg.
Update rust-alert to find deps through rustpkg
auto merge of #1253 : kmcallister/servo/rustpkg, r=jdm

Also take a rust-layers bugfix.

r? @metajack, who also started this work.

I get super nervous whenever I add a Makefile rule that starts with `rm -rf` so please check that extra carefully. :)