CMake: Clang: Be more strict wrt libclang lookup

Error out in case we find Clang C++ libraries, but do not find the Clang C
library (libclang).

Unfortunately I don't see an easy way to integrate this nicely with
FeatureSummary without rewriting all of FindClang.cmake. Let's just bail
out with FATAL_ERROR for now...

CCMAIL: dfaure@kde.org