build: adjust handling of appindicator configure options

- always #define USE_AYATANA_INDICATORS to either 0 or 1 in case
  WITH_APPINDICATOR is defined.
- previously, with meson -Dappindicator=no was not working. Fix it.
- merge the autotools variant APPINDICATOR_VARIANT with the
  --with-appindicator option. We don't need two flags to select
  a particular appindicator variant.
- add "yes" as an alias for "auto". It mirrors "no". However, I wonder
  if such kind of autodetection is really desirable. Shouldn't the user
  be required to specify the variant to use?