Compiler Setting Updates, master branch (2019.11.05.)

Extended certain compiler settings to be used with Clang as well. Since previously the the setup only used certain settings when using a GCC compiler, while it should make the same settings for Clang as well.

Most importantly whenever we use Clang as the C++ compiler, we always use GNU's gfortran for the Fortran compilation. So now the same Fortran specialisations are done for Clang as well. (It's worth noting that once the "new behaviour" of CMake 3.15 will be handled in AtlasCMake, the test will be possible to be made on CMAKE_Fortran_COMPILER_ID as well.)

Finally, made some small cleanup in the code. For instance getting rid of the use of CMAKE_COMPILER_IS_GNUCXX for instance, as it's been marked deprecated by CMake quite a while ago.

This is primarily to fix some of the compilation issues seen in the x86_64-centos7-clang8-opt nightlies, and discussed in ATLINFR-2904.