Add cmake version file

[domin144]

Add the missing cmake version file, so that software using the library can check for version compatibility.

[domin144]

cmake/OpenJPEGConfigVersion.cmake.in is copied from cmake sources, covered by 3-clause BSD license.
If this creates a licensing issue, the change could be restricted to cmake > 3.0.0 only.

They have no copyright header and cmake documentation says "they can be used as starting point to create more sophisticated custom ConfigVersion.cmake files"[1], so not sure if we should bother.

[1] https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html

[rouault]

@mayeut does that look good to you ?

[domin144]

One more doubt. I used SameMajorVersion option. If hypothetical version 3 is supposed to be compatible with any previous version If the current version is API compatible with 1.x, than AnyNewerVersion should be used instead (and BasicConfigVersion-AnyNewerVersion.cmake.in in place of BasicConfigVersion-SameMajorVersion.cmake.in).
EDIT: I mixed perspective. Version file is provided with the library, so it is it's statement of what it provides.

[rouault]

If the current version is API compatible with 1.x,

it is not

[rouault]

I believe we should just remove support for CMake < 3.0. Newer CMake versions are actually going to remove support for CMake < 3.5 soon. That way we could remove cmake/OpenJPEGConfigVersion.cmake.in

[domin144]

If dropping support for old cmake is an option, I would be really happy with that.
I think this should be a separate PR and this PR could be rebased then.

[rouault]

If dropping support for old cmake is an option, I would be really happy with that.

would you be interested in tackling that?

I think this should be a separate PR and this PR could be rebased then.

indeed

[domin144]

I will try to find some time this week to make PR removing support for cmake < 3.5.

[domin144]

Rebased.