check for and error early with cygwin or msys-target compilers

these are incapable of building libuv, but can build everything else

backport of ec1cdaf161a5ba2de83070f955552139492e9136

set XC_HOST to `uname -m`-w64-mingw32 by default for cygwin builds

backport of 123de5ac05cf8e1669f1080286c0d0229d4774b2
Document Cygwin-to-MinGW cross compilation

Replace old-and-busted MSYS1 instructions, no one should be using MSYS1 any more.

backport of 616153c7e64b32744f3ca964062e00154b3aa963

add a call to env in the git troubleshooting alias

backport of 5576084b3ca764eb9fa1e69554e7fb24d944193e