configure -cmake silently ignores -platform

1. Configure qtbase to build with CMake and clang:

   ~/dev/src/qtbase/configure -cmake -platform linux-clang
   

   The output will contain the line:

   Building for: linux-clang (x86_64, CPU features: mmx sse sse2)
   

2. Try building this:

   cmake --build . --parallel
   

   or just:

   make
   

   This will use cc1 and cc1plus, not clang and clang++.

If linux-clang cannot be used for some reason, configure should make this clear and not silently ignore what I selected explicitly.