OSX: allow ./configure with only Command Line Tools

As it is now, ROOT can't be built with Cocoa and ./configure without full Xcode
installation.  The problem is that Command Line Tools don't provide SDK the way
Xcode does.  This patch allows detecting system-wide frameworks available at
/System/... .

Signed-off-by: Fons Rademakers <Fons.Rademakers@cern.ch>