Add support to build the introspection files for GdkWin32, as done recently
in the autotools builds and clean up the NMake Makefile for building the
introspection files a bit.

For some reason, gdk_win32_display_manager_get_type() was not exported in
gdk-3.0.lib, force its export, so that the GdkWin32-3.0.gir can be built
properly with the Visual C++ builds.  This is a known problem that some
symbols in static libraries that are linked into a DLL in Visual C++, even
if they were marked with __declspec(dllexport) via _GDK_EXTERN.