clang_includes should default to (), not None
Include a copy of libclang inside xdress

At the moment, our libclang copy likely only works for clang 3.2.
However, fixing it for clang 3.3 and 3.4 will be mostly a matter
if fixing versions in the backwards compatibility #ifdef's.
Don't use -Wstrict-prototypes since the libclang extension in C++