libmstoolkit fails to cross build from source, because it does not pass
cross tools to make. Wrapping the make invocation in dh_auto_build can
be used to fix that, but it also enables concurrency and thereby turns
#1105536 fatal unless passing --no-parallel. Then, the upstream Makefile
uses non-standard variables (CC for the C++ compiler rather than CXX)
and this goes wrong when dh_auto_build overrides the toolchain. I
propose patching upstream to use standard variable names. With all of
these changes to be found in the attached patch, libmstoolkit cross
builds.

Helmut