RFC: Document Cygwin-to-MinGW cross compilation

cygwin and msys-target compilers are incapable of building libuv, but can build everything else - saves user time and frustration to check for them up front and error early. ref #8488 and #6234
that's fine. we can automatically guess that if BUILD_OS==CYGWIN, then OS==WINNT (essentially just auto-detecting that the user wants a cross-compile, since we can't to a native cygwin compile anyways)