glib2: add a macro for suppress a warning for "%p"

It is used like:

    rb_raise(rb_eArgError,
             "must specify one of them: "
             "[:stock, :icon_name, :icon_set, :gicon, :file, :pixbuf, "
             ":animation]: %p",
             RBG_PRINTF_INSPECT_VALUE(rb_options));