Fix Xcode 4.2 cc compiles

It seems like latest build of Xcode 4.2 doesn't create "-4.2" links in /usr/bin. Because of this recipes fail with "configure: error: C compiler cannot create executables" error.

Closes Homebrew/homebrew#7254.
Closes Homebrew/homebrew#7276.

Signed-off-by: Max Howell <max@methylblue.com>

Amended to work as intended, since /usr/bin/cc is actually since Xcode 4.

Amended commit message. First line must be less than 72 characters! :P