configure.ac: Clean up detection of different mjpegtoolsAPI versions.

configure.ac: Clean up detection of different mjpegtoolsAPI versions.

Original commit message from CVS:
Patch by: Mark Nauwelaerts <manauw at skynet dot be>
* configure.ac:
Clean up detection of different mjpegtoolsAPI versions.
* ext/mpeg2enc/gstmpeg2enc.cc:
* ext/mpeg2enc/gstmpeg2enc.hh:
* ext/mpeg2enc/gstmpeg2encoder.cc:
* ext/mpeg2enc/gstmpeg2encoptions.cc:
* ext/mpeg2enc/gstmpeg2encpicturereader.cc:
* ext/mpeg2enc/gstmpeg2encpicturereader.hh:
* ext/mpeg2enc/gstmpeg2encstreamwriter.cc:
* ext/mpeg2enc/gstmpeg2encstreamwriter.hh:
Streamline conditional code for evolving mjpegtools API,
optimize and fix/prevent crash in log handling, use
names/nicks for enums in the usual way andm inor updates
in code and properties/settings. Partially fixes bug #520329.