gtk+ 3.2+ uuid conflicting when build by mingw-w64

[Dongsheng Song <dongsheng song gmail com>]

In the gtk/Makefile.am file, gtk+ forced link libuuid:

libgtk_3_la_LDFLAGS += -Wl,-luuid

But in the file gdk/win32/gdkdnd-win32.c, gtk+ forced uuid initialization:

#define INITGUID

Then we got lots of FOLDERID_* redefined errors when link.

I think '#define INITGUID' should be replaced with '#undef INITGUID'.

Regards,
Dongsheng