cmake minimum should probably be bumped to 3.14

[meshula]

vfx2020 specifies boost 1.70, which is supported in cmake 3.14 and later.

We specify cmake 3.12 as a minimum. If someone is building the boost-based Imath python bindings, it won't actually be a problem, because PyImath appears to be missing a find_package statement.

Probably we need to add a find_package statement to PyImath, and bump the version.

No one has noticed this, so it's a low priority.

[lgritz]

I would be in favor of bumping cmake minimum moving forward. If you look at the VFX industry build matrix spreadsheet, you can see that a number of other projects in our ecosystem need higher than 3.12. In particular, OpenVDB and OpenTimelineIO both require 3.18, USD requires 3.14, OpenColorIO 3.13. Also, of all of the studios who have filled in their section, only one reports using older than 3.15, most are even beyond that.

I think bumping to 3.14 is conservative, I would consider going even higher. There are some nice things that come in 3.15 and 3.16. If you want to obey the "support dependencies and build tools up to 3 years back" rule, 3.17 was the one released 3 years ago (March 2020). By the time we "release" the next major version this summer, 3.18 will be 3 years old (July 2020).

Other data points: OIIO and OSL are both 3.12 still today, but I'm assuming I will bump by several releases by this summer's release season. I just haven't decided how far to move forward. But I'm certain it will be at least as far as 3.15.