gcc4x: adjust default language set

We need a more standard language set now that these formulae are available for use by Homebrew - users shouldn't be surprised by not being able to build C++ software.

This adds C++ as a default (since it's a pretty builtin expectation for Homebrew), and also enables ObjC and ObjC++ (since, surprisingly, many common packages like vim and glib require ObjC code). Refs mxcl/homebrew#24274.