Fix :gzip_only extraction

gunzip can only extract files in-place, so just shelling out to
gunzip was actually leaving the uncompressed file in Homebrew's cache,
not in the temporary directory. (It also destroyed the original
compressed file.)
superenv cc: specify llvm-g++-4.2

We previously tried to call llvm-g++ under the name `g++`,
but this is now a link to clang on Xcode 5+.

Fixes #20691.
Clarify ENV.gcc, ENV.llvm c++ compiler names