superenv defaults to servile mode. In servile mode:
* If 'gcc' is called, then 'gcc' is run (we ignore HOMEBREW_CC)
* CFLAGS (optimizations) are not applied
* ARGV is not mangled (TODO though we should apply fixes)
* -I and -L environment is still forcibly inserted.
This fixes, eg. jack which was still broken with stdenv. Jack was broken because we set CC in stdenv, and Jack has a stupid build-system. Unsetting CC allowed Jack to find and use the gcc tool it so demanded, but (previously) we would then substitute clang under its nose. The configure still failed. In servile mode (llvm-)gcc is used and Jack compiles.
In normal circumstances clang would then be inserted again during the make phase. But Jack uses the niche-wag build tool that we don't support for setting the O HOMEBREW_CCCFG flag that disables servile mode.
