FindLibClang.cmake: Add generic paths for libclang

I added some generic paths for clang's include/lib.
Arch uses /usr/include and /usr/lib.
Fedora uses /usr/include and /usr/lib/llvm.


BTW, I hate that I have to do this but I have an offtopic question:
I liked your emacs theme that you have in the screeshots, can you share it?