rtpproxy: fix build for bcg729 >1.0.1

rtpproxy: fix build for bcg729 >1.0.1

Several method signatures have changed in bcg729 1.0.1
and higher. This issue was already resolved in upstream.
In the current development branch, there is no compilable
version. Due to those changes, I have disabled linking
against bcg729 since we have bcg729 at version 1.0.2.

By disabling bcg729 the internal g729 rtpproxy API
will be used.

Signed-off-by: Jiri Slachta <jiri@slachta.eu>