This commit adds some default initial C flags for the XL compiler. The
most important is "-qhalt=e" which causes the compiler to error-out on
non-severe error messages. This is necessary to get try-compiles to
fail when bad arguments are passed to a function.
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