[Mac] Fix invalid -apple-system-font warning Pt.2
[Mac] Fix invalid -apple-system-font warning Pt.2

The fix introsuced in b9fc3ab78357c987fc43795f2197b6f75765c5e1
did not take into account that the "-apple-system-font"
may have been already detected. This can happen if the Xwt.Gtk
font backend is initialized first.

antmicro/xwt,hwthomas/xwt,lytico/xwt,mono/xwt,TheBrainTech/xwt,cra0zy/xwt,hamekoz/xwt