tap: don't print warning for apple-gcc42 failure.

This should be a temporary workaround until we get a grip on some
Mavericks/Xcode 5.0 issues with e.g. ghc and go (stuff in core
that currently needs GCC at runtime). These formulae both have
Clang fixes in development.

Things that needs GCC at compile-time should be booted from core.

References #22872.
References Homebrew/homebrew-dupes#229