[CMake] iwyu and clang-tidy not working under mac os

[Roman Wüger]

Hello,

I set CMAKE_CXX_INCLUDE_WHAT_YOU_USE and CMAKE_CXX_CLANG_TIDY. Both work as expected with CMake 3.6.2 under linux. Under Mac OS with the same sources and the same CMake version doesn't produce any output. 

On Linux clang 3.8 is used, on Mac OS it is AppleClang 7.3.0 with clang-tidy installed from homebrew.

Any hints?

Thanks in advance

Best regards
Roman