FR: Option to disable mDNS

[jinnatar]

Problem:

• Building a minimal SoapySDRServer for example as a Docker image is needlessly complicated due to the assumption that mDNS is always available.
• Not all scenarios call for mDNS, for example if alternative service discovery methods are used. In my scenario I'm actually relying on Consul service discovery.

######################################################
## Soapy Server -- Use any Soapy SDR remotely
######################################################

Server version: 0.5.2-unknown
Server UUID: a28c6e87-882b-1727-8567-000111ac1e00
Launching the server... tcp://[::]:55132
Server bound to [::]:55132
Launching discovery server... 
Connecting to DNS-SD daemon... 
[ERROR] avahi_client_new() failed: Daemon not running
Press Ctrl+C to stop the server
DNS-SD daemon disconnected...
Exiting prematurely...
Shutdown client handler threads
Cleanup complete, exiting

Proposed solution:

• Add an optional flag --disable-mdns that skips client creation and just skips straight to binding.
• Alternatively, if client creation fails, still proceed with serving, just without mDNS. This can however lead to inconsistent results if mDNS has a transient issue, so my preference is on requiring an explicit opt-in or opt-out for mDNS, whichever is preferred for expected default behavior.

[guruofquality]

try this branch: with -DENABLE_AVAHI=OFF passed to cmake #97

[jinnatar]

That seems to work at least as far as being able to compile and start the server without Avahi, which is great! I was also able to get CubicSDR to connect, so I'd say that's a working solution. The caveat is that I couldn't get Cubic to actually give me any data from the remote radio, but most likely it's my own fault. :-)