configure: Add autoconf check for libopus.

Because opus transcoding support cannot be included in the standard Asterisk
distribution, a few codec_opus implementations have popped up.  To make it
easier for people to drop in opus support in their own installations, this
patch adds configure checks for libopus.

Review: https://reviewboard.asterisk.org/r/4106/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@426234 f38db490-d61c-443f-a65b-d21fe96a405b