configure.ac: add the ability to autodetect gnome-keyring
configure.ac: add the ability to autodetect gnome-keyring

The default is still to disable it - distributors enabling this extra
dependency (which is probably desirable) should make a conscious decision
to do so.

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control