Do not predefine GTKMM_ATKMM_ENABLED for Doxygen
Do not predefine GTKMM_ATKMM_ENABLED for Doxygen

* doc/reference/Doxyfile.in (PREDEFINED): Remove GTKMM_ATKMM_ENABLED
from the list of predefined macros, since librsvgmm does not depend
on gtkmm.

GNOME/librsvgmm,GNOME/librsvgmm,GNOME/librsvgmm