Fix build of xcb_auth.c with XDMCP on WIN32

Fix build of xcb_auth.c with XDMCP on WIN32

Fix a redefinition problem which shows up when building for _WIN32 and
libXdmcp is installed, so HASXDMAUTH is enabled

It seems this is a special place in xcb as it uses other X11 library headers here

If HASXDMAUTH is defined, include the wrapped windows.h before any header which
includes it unwrapped, to avoid conflicts with types defined in X headers

We need to include config.h and check HASXDMAUTH to avoid an unconditional dependency
on x11proto headers

In file included from install/include/X11/Xdmcp.h:19:0,
                 from git/xcb/libxcb/src/xcb_auth.c:52:
install/include/X11/Xmd.h:120:14: error: conflicting types for 'INT32'
/usr/i686-pc-mingw32/sys-root/mingw/include/basetsd.h:54:13: note: previous declaration of 'INT32' was here
install/include/X11/Xmd.h:143:15: error: conflicting types for 'BOOL'
/usr/i686-pc-mingw32/sys-root/mingw/include/windef.h:234:17: note: previous declaration of 'BOOL' was here

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>