Enable policy 77 if possible.

[Anteru]

When using OpenEXR as a submodule, you currently can't set a variable
outside of it as the option will override it (so you need to set the
option.) With policy 77 in place, this works as expected. (Same change as just submitted for OpenEXR, see AcademySoftwareFoundation/openexr#961)

Signed-off-by: Matthäus G. Chajdas [email protected]

[meshula]

Same question as on the corresponding OpenEXR PR - do we need to also bump CMake minimum to 3.13?

[lgritz]

@meshula Certainly not for 3.0! We should not be changing dependency requirements at this stage.

Let's get this release done, and then as a separate matter, I think the whole TAC should decide if we want to make a policy decision across all projects about what is a reasonable minimum CMake. For a number of reasons, I would support bumping from 3.12 to something newer, but let's do it by consensus and with a principled reason for which version specifically. There are also some really nice things in 3.15, such as the -B an -S arguments to cmake that eliminate the confusing directory dance.

EDIT: -B and -S are in 3.13. But I still think this is a TAC topic.

[lgritz]

I approve the changes, but I feel less strongly that it should be merged before the release.

[cary-ilm]

This seems innocuous enough to include, and easy enough to back out of if it causes a problem. I vote for merging, then address the cmake version issues later.

[Anteru]

Thanks!