Reduce libebook dependency when not compiling the BlueZ backend

Debian unstable is currently stuck with Evolution 3.8, due to the
number of ABI transitions required to get from here to there. Folks
does not appear to need a newer version if the optional BlueZ backend
is not installed, and Debian doesn't currently enable that backend
anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=726980
EDS test-cases: be backwards-compatible with Evolution 3.8

The D-Bus API version changes between releases. We want all known
versions to be activatable.

https://bugzilla.gnome.org/show_bug.cgi?id=726980