samba: make installation of smbd configurable

It may be desirable to compile samba but not actually install
the smbd binary. This is the case if you need libsmbclient but
don't want to run a samba server.

Make this possible by allowing to disable installation of smbd.

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