- Simplify CMake GCC and Clang checking.

Introduce the variable 'ZD_CMAKE_COMPILER_IS_GNUC(XX)_COMPATIBLE' and replace any occurrence of '"${CMAKE_C(XX)_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_C(XX)_COMPILER_ID}" STREQUAL "Clang"' with it. This makes it possible to add more GCC compatible compilers in just one place.