Configurable `builds` via select env settings

[springmeyer]

Mason users may need to rebuild packages from source using a different compiler than was used for the binary packages. To enable this we'll need to expose the CXX and CC option rather than hardcoding in the mason.sh.

This will need to play well with specific scripts that have specific compiler needs (like LLVM compiles itself with a specific clang++ version: should it respect the override?)

also related: #492

refs https://github.com/mapbox/carmen-cache/issues/117#issuecomment-369051742