Update configure.ac to fix the enable_gstreamer summary
The current version (building using autotools) displays Enable GStreamer-based RTP: no even when --enable-gstreamer is in use
That's because the check is done using AS_IF([test "x$HAVE_GSTREAMER" = "x1"], ENABLE_GSTREAMER=yes, ENABLE_GSTREAMER=no)
or the code set HAVE_GSTREAMER to 'yes' and not '1'. The patch here fixes the logic by using 1/0 instead of yes/no (as it's done for the others options)