The installation is performed with the zfs 0.7.9 packages just released from the debian backports repositories.
The OMV-Extras.org Testing repository is disabled.
The only additional repository I have activated is the debian contrib which does not affect the zfs installation anyway.
Successive compilation errors (lack of kmod spl devel) remain but the process still terminates with working zfs.
@ryecoaaron: I'm wondering when it will be possible to release version 4.0.3 of openmediavault-zfs which is now in testing.
I attach the command line log.