Enable usage of system-provided iLBC library.

The WebRTC version of the iLBC codec is now package as a library and is
available on some platforms. This patch allows codec_ilbc to be built against
that library if it is present.

Review: https://reviewboard.asterisk.org/r/1964/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370407 65c4cc65-6c06-0410-ace0-fbb531ad65f3