Make sure gcc_selected() returns something useful on Xcode 5.

On Xcode 5, /usr/bin/gcc is no longer a symlink but actually clang.
This causes Fink::Services::gcc_selected() to return "0" which prevents
fink from installing. It would also break installing packages with a
GCC field. This patch always returns "4.2" when gcc is clang.
Merge pull request #85 from danielj7/xcode-5-fix

Make sure gcc_selected() returns something useful on Xcode 5.