win32: mkdir -p win32 needed for out of tree builds

if cross compiling clutter using mingw using an out of tree build
directory then a pre-requisite for creating the resources.o file
containing the transparent cursor is for the win32 directory itself to
be created at $(top_builddir)/clutter/win32.

clutter/Makefile.am

@@ -412,6 +412,7 @@ if SUPPORT_WIN32
 # won't let you link against the .o directly because it wants you to
 # link against libtool objects for dynamic libraries.
 .rc.o :
+	mkdir -p win32
 	$(WINDRES) -I$(srcdir)/win32 $< $@
 
 win32/resources.o : $(srcdir)/win32/invisible-cursor.cur