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
Update mac platform submodules.
Fix clean-servo to include clean-style.
Update dependencies.
Port gfx crate to new CF bindings.
auto merge of #1260 : metajack/servo/kmcallister-rustpkg, r=pcwalton

Rebased from @kmcallister's branch to fix mac platform issues.