sfml: Use libglvnd on FreeBSD and Linux

[jwillikers]

The libglvnd package provides the necessary opengl/system dependency. The libglvnd package is a proper, non-system Conan package.

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 1 (64e9e60b01dcb459649fa0649215154c1a70ca8d):

• sfml/2.6.1:
  All packages built successfully! (All logs)

• sfml/2.6.0:
  All packages built successfully! (All logs)

• sfml/2.5.1:
  All packages built successfully! (All logs)

---

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All green in build 1 (64e9e60b01dcb459649fa0649215154c1a70ca8d):

• sfml/2.6.1:
  All packages built successfully! (All logs)

• sfml/2.5.1:
  All packages built successfully! (All logs)

• sfml/2.6.0:
  All packages built successfully! (All logs)

[franramirez688]

@jwillikers thanks a lot for the PR!

I was reading about the libglvnd. I did not know much about it to be honest (apart from the Changelog lines mentioned in this PR).
Now, I was wondering if we should set the CMake variable mentioned here: https://github.com/SFML/SFML/blob/d6e1961112c29eee7f60128c29ed699795505167/src/SFML/Window/CMakeLists.txt#L294-L299

[jwillikers]

@jwillikers thanks a lot for the PR!

I was reading about the libglvnd. I did not know much about it to be honest (apart from the Changelog lines mentioned in this PR). Now, I was wondering if we should set the CMake variable mentioned here: https://github.com/SFML/SFML/blob/d6e1961112c29eee7f60128c29ed699795505167/src/SFML/Window/CMakeLists.txt#L294-L299

Yes, I think so. That variable is a built-in one for CMake's FindOpenGL module. I've already set it in the generate method in this PR.

Also, it should be noted there's been further discussion on using libglvnd in PR #23439. I've opened PR's for every recipe that's up-to-date and uses opengl/system to migrate to the libglvnd package. I think there's only one that I'm aware of that is failing in CI, but that's because it uses its own homebrew build-system.

[jcar87]

Closing pending discussion at #23649