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

Library/Homebrew/cmd/tap.rb

@@ -67,10 +67,13 @@ module Homebrew extend self
           tapped += 1
         else
           to = to.realpath if to.exist?
+          # Whitelist gcc42 temporarily until Mavericks/Xcode 5.0 issues are resolved.
+          unless to.tap_ref == 'mxcl/master/apple-gcc42'
             opoo "Could not tap #{Tty.white}#{from.tap_ref}#{Tty.reset} over #{Tty.white}#{to.tap_ref}#{Tty.reset}"
           end
         end
       end
+    end
 
     HOMEBREW_LIBRARY.join("Formula/.gitignore").atomic_write(ignores.uniq.join("\n"))