autogen.sh: don't use USE_GNOME2_MACROS (deprecated)
autogen.sh: don't use USE_GNOME2_MACROS (deprecated)

There was a warning during the build:

***Warning*** USE_GNOME2_MACROS is deprecated, you may remove it from
autogen.sh

GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview