autogen: fix check for gtkdocize and autoreconf.
autogen: fix check for gtkdocize and autoreconf.

If gtkdocize or autoreconf programs were not found, then the autogen.sh
script would fail to report that correctly because test -z was not passed
any argument (empty string "" in this case).

01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi