Define WIN32_LEAN_AND_MEAN in Windows port

Define WIN32_LEAN_AND_MEAN in Windows port

<windows.h> somewhat cloyingly attempts to include <winsock.h>
by default, which causes problems if the rest of the program
is trying to use the incompatible <winsock2.h>.  The Windows
sysdep header attempts to prevent this by forcibly defining
the winsock header guard macro, so that it will not be included.
However, this does not work on MinGW because it uses a different
guard macro name.

This patch changes the code to instead define WIN32_LEAN_AND_MEAN,
which is a more portable way to ensure that <winsock.h> will not
be included.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=71405
Reviewed-By: Ralf Habacker <ralf.habacker@freenet.de>