Not using the system libclang by default

We used to do this but it was unsafe, as issue #167 proves. YCM has to give
libclang an include to YCM's copy of clang system headers (again, see issue #167
for details) and those headers may not be valid for a newer libclang.

If the user really wants to user the system libclang, then he can just always
call cmake himself. The installation guide in the README goes to great lengths
to explain the simple process of building YCM "by hand".

Fixes #167.