Fix build on mac gcc 4.2. #380

This adds clang detection to the Makefiles in skia and rust-azure and only adds `-Wno-c++11-extensions` when building with clang. It also disables the check in mfbt that we are building with gcc 4.4 or greater so that building on mac with llvm-gcc 4.2 works.

Depends on https://github.com/mozilla-servo/rust-azure/pull/14 and https://github.com/mozilla-servo/skia/pull/10
Fixes #380