superenv: find Homebrew's gcc-4.2

If we're using a homebrewed gcc-4.2, xcrun may fail to find it (or, worse, find superenv's shim instead). Explicitly add it to the PATH and search all path elements for the requested tool.

This also fixes a bad assumption I'd noticed where superenv calls gcc-4.2 as "gcc", which is wrong on several Xcode versions!

Thought I'd put it out there to see how we felt about superenv searching the entire PATH. We could extend this later to let us support non-Apple GCCs.