Take notice of USE_DEBUG_RUNTIME also in set_wntx64.mk.

Don't unset USE_DEBUG_RUNTIME and don't hardcode the non-debugging
msvcprt.lib when building the Explorer extension.

Undefine OSL_DEBUG_LEVEL in its source files that would otherwise drag
in the diagnosing functions from sal. We don't link the Explorer
extension with sal.

As such it might not make much sense to actually *use* a shell
extension built with assertions in the C++ library, against the
debugging C/C++ runtime. You don't want assertions to fire in
Explorer, do you? On the other hand, one could be debugging an
Explorer extension remotely. Maybe even locally, I don't know if some
lower levels of Windows get upset if Explorer is unresponsive due to
being debugged, or something.

But anyway, the main point for now is to make an --enable-dbgutil
build succeed.

Alternatively we could also make sure USE_DEBUG_RUNTIME is unset in
*all* makefiles in shell.