Fix huge bug in makefile prevent rust from building with debugging or with -O.
Fix nss/nspr depdency chain.
Build rustc with debugging enabled.
Add *.dSYM to clean targets.
Make rust submodules depend on rust.
auto merge of #972 : metajack/servo/parbuild-bugs, r=kmcallister

This fixes several makefile issues around parallel builds as well as some general cleanup.