Add support for cygwin

[rasky]

Fixes #62

[darealshinji]

It should be noted that the check for __MINGW64__ is also not needed because __MINGW32__ is defined on both 32 and 64 bit targets:

$ echo | ~/opt/mxe/usr/bin/x86_64-w64-mingw32.static-gcc -E -dM - | grep MINGW
#define __MINGW32__ 1
#define __MINGW64__ 1

[darealshinji]

This check is also required for clang-cl. Is the .type directive generally not supported by Windows targets?