Create platform information modules Platform/Darwin-Intel-(C|CXX).cmake
and helper module Platform/Darwin-Intel.cmake. Teach existing module
Platform/Darwin-Intel-Fortran.cmake to use the helper too. Move
information from Platform/Darwin-icc.cmake into these files and drop
information already in Platform/Darwin.cmake to avoid duplication.
The linker flags for setting the compatibility and current versions of
libraries on Darwin are set for the supported Fortran compilers.
Set CMAKE_Fortran_CREATE_SHARED_LIBRARY for the NAG Fortran compiler to
have no space after <SONAME_FLAG> so the NAG compiler can parse the
argument correctly.