Various buildsystem improvements:

- Find and use execinfo.h on non-Linux systems, thanks to sph for providing the patch
- Make OpenSSL and D-Bus support optional (-DWITH_OPENSSL=OFF, -DWITH_DBUS=OFF)
  Those options default to ON, which means they're enabled if the needed libs are found.
- For SSL being enabled in Quassel, now both OpenSSL headers/libs and SSL support in Qt
  must be present. This now sets -DHAVE_SSL which should be preferred over -DQT_NO_OPENSSL
  in the future (not changed in code yet).