Pop-Rock Depeche Mode: "Construction Time Again.'' (Sire/Warner Bros. 23900-1) - Electro pop or synth-rock (whatever you want to call it) is still somewhat of a novelty. It may take a while before music played mostly, or entirely, on synthesizers is more widely heard. But judging from ''Construction Time Again,'' the British band Depeche Mode is doing little to help it. If nothing else, this record gives the listener a text-book example of the many uses and strange effects a synthesizer produces. Unfortunately, the melodies that these machines pump out are not catchy enough to carry the band's sterile-sounding instrumentals and somewhat puzzling lyrics. ''Shame,'' a cut reveling in the world of guilt and forgiveness, has a fascinating theme, but is musically empty. Much the same can be said for most of the music here. By the end of the record, the strange sounds become a bit tedious. If you are an electric whiz kid, this record may be for you. Otherwise, leave it on the shelf.