Check for and include libavutil.h to avoid unresolved symbols.

The a52 plugin uses av_free(), but does not check for libavutil
availability. Patch configure.in to check for the module, and include
libavutil.h.

Signed-off-by: Jordi Mallach <jordi@debian.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Autotools