Update FindLibClang.cmake

Based on https://github.com/Sarcasm/irony-mode/blob/develop/cmake/FindLibClang.cmake#L20

The right thing to do would be to have a _common_ FindLibClang.cmake (e.g. /usr/share/cmake-2.8/Modules/FindLibClang.cmake) that can be used by anything that needs libclang.
build instructions for building on Ubuntu 13.10