Manual cleanup of cmake code:

Introduce new primitives to simplify lots of tests for GNU and/or Clang
MY_COMPILER_IS_GNU
MY_COMPILER_IS_CLANG
MY_COMPILER_IS_GNU_OR_CLANG

Cosmetic changes in CMakeLists.txt files modified because of the above.

Change-Id: I8e37fe43096c9f4e05fc8639607a27fed8c8241f