Fixes and new functionality for '-t'; also a small build fix

Hi,

This branch includes a few small things:
  - first, a build fix -- xsel wasn't compiling for me (on Debian jessie) due to not getting `-lX11` passed to the link command
  - second, some fixes to `setitimer(2)` usage -- depending on the timeout value it could fail with `EINVAL`, and using virtual time instead of real time was, I think, not what was really intended
  - finally, a new feature: in `--input` mode, the `-t` flag now specifies a duration after which the daemon exits.  This is useful for an "ephemeral" copy operation (e.g. of a password you don't want hanging around in your clipboard unnecessarily).

Note that I am completely unfamiliar with X11 programming, so I hope i haven't done anything horribly wrong in the third patch -- for example, if the timer expires and delivers `SIGALRM` while the daemon's in the middle of doing some selection operation, will that leave things in a weird state?  (Hopefully not, but I don't know.)  Basic testing indicates that it does what I was aiming for, though.


Thanks,
Zev Weiss