Doesn't work with stated cmake minimum (3.7)

[jagerman]

I can't build with the cmake 3.7 (currently stated as the minimum required) because $<IF is not supported in cmake 3.7 (looks like it was added in 3.8).

Bumping the minimum to 3.8 would "solve" this issue, but not in a particularly helpful way. Instead I wonder: is there more than just the $<IF generator expressions that require a newer cmake? It would be very nice to back this dependency down a bit to at least 3.5 so that packages depending on date.h could be built on xenial linux systems, which are still fairly common. (To be clear: I'm happy to submit a PR to do it, just trying to gauge interest in supporting it).