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
where? the issue is when you have a long PATH, git likes to fork into every entry in it for some reason, the workaround is temporarily setting PATH to only `/usr/bin` for the purposes of calling git