Merge branch 'jc/not-mingw-cygwin'

We have been using NOT_{MINGW,CYGWIN} test prerequisites long before Peff invented support for negated prerequisites e.g. !MINGW and we still add more uses of the former. Convert them to the latter to avoid confusion. * jc/not-mingw-cygwin: test prerequisites: enumerate with commas test prerequisites: eradicate NOT_FOO