message: cmake: add include-what-you-use
  Without using or enforcing it yet.
diff: @@ -29,3 +29,12 @@ if(ENABLE_CLANG_TIDY)
  message(SEND_ERROR "clang-tidy requested but executable not found")
  endif()
  endif()
  +
  +if(ENABLE_INCLUDE_WHAT_YOU_USE)
  + find_program(INCLUDE_WHAT_YOU_USE include-what-you-use)
  + if(INCLUDE_WHAT_YOU_USE)
  + set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE ${INCLUDE_WHAT_YOU_USE})
  + else()
  + message(SEND_ERROR "include-what-you-use requested but executable not found")
  + endif()
  +endif()
  \ No newline at end of file
  