qualify-clang: improve CMake test

Peter Wurequested to merge
Lekensteyn-guest/llvm-toolchain:cmake-clang-qualify into 5.0
  • Explicitly check that both the default CMake find_package and the explicit version-specific check works as intended.
  • Set CC/CXX to avoid requiring an additional compiler to be installed.
  • Add comments with package suggestions.

Addresses an issue found while testing 5.0.2 with Ubuntu 14.04 (see !2 (comment 23629)).

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