fix build on stable distros
Many stable distros don't have the fairly new glib 2.68.0 version, so installing libxfcewindowing brings in a long and invasive dependency chain. All we really need that for is g_memdup2(), which isn't really necessary for us.
This can be solved by adding a tiny inline wrapper if glib isn't new enough - the problem that g_memdup2() solves isn't really relevant here (it's just a few tiny icons - no severl GBs large buffers).
Closes: https://gitlab.xfce.org/metux/libxfce4windowing/-/issues/3 Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net