samba: explicitly link with -pthread if avahi support is enabled

The avahi-client library makes use of threads. We need to link the
smbd binary with -pthread if avahi support is enabled. Otherwise the
binary will crash at startup.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>