temp patch for win32 readdir issue

Ok, so I know this is pretty ugly. But there is a bug in mingw (current
release - it's fixed in the snapshot version) with regards to readdir()
(see previous mails on win32-hackers). Basically, it doesn't set errno
correctly.

This patch will catch this error with regards to the readdir(), wrap a
change in #ifdef WIN32, and emit a warning at compile time. This is a
temporary measure until mingw releases a version that has this fixed.

Applying this will make it easier on those who want to compile
postgresql on win32, since it removes a manual step. Which is why I'm
giving it a shot posting it here even though it's ugly.

//Magnus