build: add -DMINIUPNP_STATICLIB for new version

libminiupnpc changed their required static define to the much more sane
"MINIUPNP_STATICLIB". Sadly, they don't respect the old "STATICLIB" for
back-compat. Define them both since the old one didn't seem to be conflicting
anywhere.

Also go ahead and split out the cppflags so that they can be applied only where
they're needed. This will help us to build dll's from our libs without having
their import/export declspecs poisoned.