Its alternative values as used by OOo is irrelevant to us as we don't
intend to support building using MinGW on Windows itself. To us, MinGW
always means cross-compilation. For us it is enough to look at
$(OS)$(COM), and WNTGCC always implies cross-compilation.

(OOo on the other hand attempts to support use of the Cygwin gcc with
the -mno-cygwin option (which is practically considered an obsolete
option), the normal MinGW compiler (but still from Cygwin), but not
cros-compilation.)