Don't build MVerb UI on Windows

plugins/Makefile.MVerb.mk

@@ -12,6 +12,11 @@ ifeq ($(OBJS_UI),)
 HAVE_DGL = false
 endif
 
+ifeq ($(WIN32),true)
+# FIXME: make OpenGL >= 2 work with mingw
+HAVE_DGL = false
+endif
+
 # --------------------------------------------------------------
 # Basic setup