More INSTALL updates.

- We now support Gtk+-3, Gtk+-2, and Qt (Gtk+-1 is long dead).
- Sync several of the configure options to what ./configure actually does
  (don't bother listing features which default to disabled though).
- Don't mention sysconfdir since manuf doesn't go there any more (it goes
  in datadir).
- Remove special install instructions for an ancient version of libpcap.
- Point users to the Q&A site.

Change-Id: I96a9e13fcf4225c526c5d9b536425b2d20e659f6
Reviewed-on: https://code.wireshark.org/review/3566
Reviewed-by: Evan Huus <eapache@gmail.com>