superenv: set ENV cc/cxx based on actual compiler

This needs more testing and is more for discussion than rapid merging.

Go is one package this immediately benefits, Qt may be another (I need to do more testing there but I recall it also behaves differently when it detects Clang).

The superenv defaults seem a bit like IE putting Mozilla in the user-agent; it made sense initially but at this point it's unhelpful to upstreams who want to adjust behavior depending on the compiler used.