superenv: always respect HOMEBREW_CC

The current version of the superenv compiler shims only respect the HOMEBREW_CC compiler selection if the compiler is called under the names `cc`, `c99`, `c89` or `c++`, and otherwise superenv is falling back to the name under which the tool was called. As a result we're seeing some build scripts with hardcoded `gcc` invocations using the actual compiler by that name even though we intercepted it via a symlink. This changes the default fallback condition to *always* use HOMEBREW_CC. Hopefully this should clear up some Xcode 5 problems.