fix OSX compile issue due to 68e81a6

68e81a6 should be improved/reverted. Due to the long include chain ending
in ardour_ui.h  every Lua related change now forces a complete GUI
recompile (instead of just 2,3 files)

gtk2_ardour/cocoacarbon.mm

@@ -29,6 +29,8 @@
 #undef verify
 #endif
 
+#undef Nil // ardour_ui.h ->  window_manager.h -> lua_script_manager.h -> ardour/luascripting.h -> ... lua  uses Nil
+
 #include "ardour_ui.h"
 #include "actions.h"
 #include "opts.h"