* If only Ayatana Indicators are present in the build env,
    build against them.
  * If only Ubuntu Indicators are present in the build env,
    build against them.

  * If both Indicator implementations are present in the build env,
    then we build against Ayatana Indicators. A build against
    this or that implementation can be enforced by configure options
    (--with-ayatana-indicators=yes or --with-ubuntu-indicators=yes).

  * If either implementation is present, but the other -with-*-indicator
    configure option is given, an error is thrown at configure time.
  * No Indicator implementation present in the build env, of course,
    also throws an error at configure time.

fixes https://github.com/mate-desktop/mate-indicator-applet/issues/9

and require mate-panel >= 1.17

they don't have .so files anymore, only the service binary,
and they're loaded from /usr/share/unity/indicators

they're supported only with libindicator3 >= 12.10

also this adds build-dep on libido (hmm, what Freud would say?)

will be handy for changing volume in indicator-sound with mouse scroll