IBM rebranded its VisualAge compiler to XL starting at version 8.0. We
use the compiler id "XL" for newer versions and "VisualAge" for older
versions. We now also recognize the "z/OS" compiler, which is distinct
from XL.
The CMAKE_Fortran_DEFINE_FLAG value applies to the IBM Fortran compilers
on all platforms. This moves the setting to the platform-independent
compiler information file.