Makefile: pass --output to glib-mkenums
Makefile: pass --output to glib-mkenums

This avoids hitting https://gitlab.gnome.org/GNOME/glib/issues/1430. It
is safe to do this because glib-mkenums performs the "write to temporary
file, then rename" dance.

pwithnall/dunfell,pwithnall/dunfell