* 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.