some distros already packaging clang 10 (checked fedora and arch) no longer
ship all the individual libclang*.so component libraries.
Instead, clang from 9.0 onwards provides a new lib, libclang-cpp.so, which
includes everything we need.

Tell cmake to use it if the individual libraries are no longer found.
(Build-wise, if both are present it is more efficient to use the individual
components so keep these first)