Clarify configure GStreamer handling

Currrently --disables-gstreamer disables GStreamer API 1.0 support
and --enable-gstreamer-0-10 enables GStreamer API 0.10 support. To
build with GStreamer API 0.10 you need to --disable-gstreamer and
--enable-gstreamer-0-10, which doesn't seem logical.

But actually it's possible to build both avmedia GStreamer backends,
so this changes:

 * --disable-gstreamer to --disable-gstreamer-1-0
 * configure variables to include the GStreamer API version "1_0"
 * adapts the configure help text

Change-Id: Icffd8cfb9d80b3021e290675e7c9644c2a31fce8
Reviewed-on: https://gerrit.libreoffice.org/11912
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>