Build some dependencies with rustpkg

Also take a rust-layers bugfix.

In collaboration with Jack Moffitt.
auto merge of #1253 : kmcallister/servo/rustpkg, r=metajack

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. :)