janus-gateway: make voicemail plugin optional

janus-gateway: make voicemail plugin optional

Currently janus-gateway's voicemail plugin is only built if
libogg is selected, and even then there is no way to disable the plugin from
being built. This causes a example config file to be placed on
the target filing system that the user may not need or want, creates a 33K .so
file on the target directory, and janus will also generate 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 voicemail plugin optional, removes the check for libogg,
adds libogg as a dependency to the voicemail plugin, and selects the libogg
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>