compat/poll: prepare for targeting Windows Vista

Windows Vista (and later) actually have a working poll(), but we still
cannot use it because it only works on sockets.

So let's detect when we are targeting Windows Vista and undefine those
constants, and define `pollfd` so that we can declare our own pollfd
struct.

We also need to make sure that we override those constants *after*
`winsock2.h` has been `#include`d (otherwise we would not really
override those constants).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/poll/poll.h

@@ -21,6 +21,21 @@
 #ifndef _GL_POLL_H
 #define _GL_POLL_H
 
+#if defined(_WIN32_WINNT) && _WIN32_WINNT >= 0x600
+/* Vista has its own, socket-only poll() */
+#undef POLLIN
+#undef POLLPRI
+#undef POLLOUT
+#undef POLLERR
+#undef POLLHUP
+#undef POLLNVAL
+#undef POLLRDNORM
+#undef POLLRDBAND
+#undef POLLWRNORM
+#undef POLLWRBAND
+#define pollfd compat_pollfd
+#endif
+
 /* fake a poll(2) environment */
 #define POLLIN      0x0001      /* any readable data available   */
 #define POLLPRI     0x0002      /* OOB/Urgent readable data      */