Add XCode system library path to fix --system-libclang building

At least on my Os X 10.9.1 installation, the libclang resides in:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib

The directory:

/Library/Developer/CommandLineTools/usr/lib

Does not even exist on my system, that might be only for Xcode 4 installations.