cmake, sa: enable unix sockets, if HAVE_UNIXSOCK is undefined

[fAuernigg]

Unix sockets are turned on per default in cmake and are used in header re_sa.h. This PR enables the unix socket flag, if not explicitly defined during builds of external libs using re_sa.h.

Edit by @cspiel1 : Relates to baresip/baresip#2402

[fAuernigg]

I agree that positive logic would be better but is not a solution here.

If sa struct is used via libre api's but from external libaries, a missing "HAVE_UNIXSOCK" produces a very hard find segv: 'stack smashed" in my case.
If a missing HAVE_UNIXSOCK would lead to a compile time error this would be ok for me. A not debug'able crash at runtime due to a missing definement or updated lib re default value for HAVE_UNIXSOCK is really ugly.

I had to add printfs arround the code smashing the stack to finally find out, which definement and which default value was missing in my library.

[sreimers]

Looks good to me (ignore my deleted comment)

[fAuernigg]

This PR handles 3 states of USE_UNIXSOCK / HAVE_UNIXSOCK.
ON, OFF, undefined (in third party libs using re_sa.h but having no clue of the default value)