Factor duplicate information out of Compiler/XL-<lang>.cmake modules
into a macro in a new Compiler/XL.cmake module. Invoke it from the
per-language files to produce the original settings.
Since commit e1729238 (Add initial XL C compiler flags for safer builds,
2009-09-30) CMake sets the initial XL C flags to include "-qthreaded"
and "-qhalt=e". Do the same for C++ and Fortran with this toolchain.
This moves platform-independent XL compiler flags into separate
"Compiler/XL-<lang>.cmake" modules. Platform-specific flags go in
"Platform/<os>-XL-<lang>.cmake" modules.
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.
Brad King