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
`uname` and therefore `BUILD_OS` are `CYGWIN_NT-#.#` so it should use a findstring not an exact match, but yeah. Looking at https://github.com/JuliaLang/julia/blob/49083be6e7c4ceaa3f457afd1d126084c18615d7/Make.inc#L163-L164 we already set `OS` to `WINNT` for any mingw-containing values of `XC_HOST`. I'll put in a cygwin-only default setting of `XC_HOST` there at line 155.

For posterity if anyone's curious why libuv won't build for Cygwin or msys targets, see https://github.com/joyent/libuv/issues/845#issuecomment-50260637 - the Cygwin posix dll doesn't appear to have any async io capabilities.