configure: suppress some warnings when debug is disabled
configure: suppress some warnings when debug is disabled

Warnings about unused variables should be suppressed if core has the
debug system disabled.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=676824

surround-io/gst-rtsp-server,barisdemiray/gst-rtsp-server,surround-io/gst-rtsp-server,hsteinhaus/gst-rtsp-server,Lachann/gst-rtsp-server,Lachann/gst-rtsp-server,Lachann/gst-rtsp-server,thaytan/gst-rtsp-server,thaytan/gst-rtsp-server,hsteinhaus/gst-rtsp-server,barisdemiray/gst-rtsp-server,thaytan/gst-rtsp-server,barisdemiray/gst-rtsp-server,hsteinhaus/gst-rtsp-server,surround-io/gst-rtsp-server