Annotate Makefile with hints about building under Windows

You can build subsurface with MinGW. It requires installing lots of random
packages, plus some things still don't work. One is that xml2-config
appears to be missing. So this annotates the Makefile to tell a person
building under Windows how to work around this. But we can't make this
platform conditional as this workaround is hardcoding the install path.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>