janus-gateway: make audio bridge plugin optional

janus-gateway: make audio bridge plugin optional

Currently janus-gateway is built by default with the audio bridge plugin.
This causes a example config file to be placed on the target filing system
that the user may not need or want, creates a 119K .so
file on the target directory, adds the dependency of opus for this
plugin, and janus will also create a warning when ran if the plugin does
not have a matching configuration file setup by the user, as it will not use
the example config file by default.

This patch makes the audio bridge plugin a option, removes the check for opus,
adds opus as a dependency to the audio bridge plugin, and selects the opus
package automatically when the user selects the plugin.

Signed-off-by: Adam Duskett <aduskett@codeblue.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>