Make sure the MINGW_FOO environment variables get set and propagated
to the build environment also in the MinGW cross-compilation case. The
OOo code used to do that for MinGW natively on Windows (under
Cygwin). (Which we don't intend to support.)

Now, whether the *use* of these variables in the various makefiles etc
is relevant any more remains to be seen. I suspect all that might well
be unnecessary, as we after all are capable of cross-build the code
using MinGW just fine currently with none of these MINGW_FOO being
set.

One place where at least MINGW_GCCDLL and MINGW_GXXDLL is needed,
though, is in scp2. We presumably do want to include these DLLs (the
shared libgcc and libstdc++) in the installation set, to the extent
the scp2 stuff can be used still in a MinGW cross-build context.