configure.ac: rework similarly to other modules

configure.ac: rework similarly to other modules

Original commit message from CVS:

* configure.ac:
rework similarly to other modules
* ext/a52dec/gsta52dec.c:
* ext/amrnb/amrnb.c:
* ext/dvdnav/dvdnavsrc.c:
* ext/dvdread/dvdreadsrc.c:
* ext/lame/gstlame.c:
* ext/mad/gstid3tag.c:
* ext/mpeg2dec/gstmpeg2dec.c:
* ext/sidplay/gstsiddec.cc:
* gst/asfdemux/gstasf.c:
* gst/dvdlpcmdec/gstdvdlpcmdec.c:
* gst/dvdsub/gstdvdsubdec.c:
* gst/iec958/ac3iec.c:
* gst/mpegaudioparse/gstmpegaudioparse.c:
* gst/mpegstream/gstmpegstream.c:
* gst/realmedia/rmdemux.c: (plugin_init):
use the correct defines