Also change elements factory names to avenc_*, avdec_*,
avmux_* and avdemux_*. The actual filenames, type names,
function/variable names are not touched to make merging
with 0.10 easier at this point. Once 0.10 is not supported
anymore everything should be renamed.
Original commit message from CVS:
* ext/ffmpeg/Makefile.am:
* ext/libpostproc/Makefile.am:
* ext/libswscale/Makefile.am:
Don't install static libs for plugins. Fixes#550851 for -ffmpeg.