CMake warning about LEGACY and GLVND libraries for OpenGL on Linux

[kallaballa]

System Information

OpenCV version: 4.x
Operating System / Platform: openSUSE Tumbleweed (20221102) / Linux
Compiler & compiler version: g++ (SUSE Linux) 12.2.1 20221020 [revision 0aaef83351473e8f4eb774f8f999bbe87a4866d7]

Detailed description

When invoking cmake to configure the build of OpenCV there is a warning when configured with OpenGL:

CMake Warning (dev) at /usr/share/cmake/Modules/FindOpenGL.cmake:315 (message):
  Policy CMP0072 is not set: FindOpenGL prefers GLVND by default when
  available.  Run "cmake --help-policy CMP0072" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

  FindOpenGL found both a legacy GL library:

    OPENGL_gl_LIBRARY: /lib64/libGL.so

  and GLVND libraries for OpenGL and GLX:

    OPENGL_opengl_LIBRARY: /lib64/libOpenGL.so
    OPENGL_glx_LIBRARY: /lib64/libGLX.so

  OpenGL_GL_PREFERENCE has not been set to "GLVND" or "LEGACY", so for
  compatibility with CMake 3.10 and below the legacy GL library will be used.
Call Stack (most recent call first):
  modules/highgui/cmake/detect_gtk.cmake:40 (find_package)
  modules/highgui/cmake/init.cmake:35 (include)
  modules/highgui/cmake/init.cmake:39 (add_backend)
  cmake/OpenCVModule.cmake:298 (include)
  cmake/OpenCVModule.cmake:361 (_add_modules_1)
  cmake/OpenCVModule.cmake:385 (ocv_glob_modules)
  CMakeLists.txt:945 (ocv_register_modules)
This warning is for project developers.  Use -Wno-dev to suppress it.

Steps to reproduce

Reproducer:

git clone -b 4.x https://github.com/opencv/opencv.git
mkdir opencv/build
cd opencv/build
cmake -DWITH_OPENGL=ON ..

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files (videos, images, onnx, etc)

[kallaballa]

From the cmake help:

The ``FindOpenGL`` module provides an ``OpenGL::GL`` target and an
``OPENGL_LIBRARIES`` variable for projects to use for legacy GL interfaces.
When both a legacy GL library (e.g. ``libGL.so``) and GLVND libraries
for OpenGL and GLX (e.g. ``libOpenGL.so`` and ``libGLX.so``) are available,
the module must choose between them.  It documents an ``OpenGL_GL_PREFERENCE``
variable that can be used to specify an explicit preference.  When no such
preference is set, the module must choose a default preference.

CMake 3.11 and above prefer to choose GLVND libraries.  This policy provides
compatibility with projects that expect the legacy GL library to be used.

The ``OLD`` behavior for this policy is to set ``OpenGL_GL_PREFERENCE`` to
``LEGACY``.  The ``NEW`` behavior for this policy is to set
``OpenGL_GL_PREFERENCE`` to ``GLVND``.

This policy was introduced in CMake version 3.11.  CMake version
3.24.2 warns when the policy is not set and uses ``OLD`` behavior.
Use the ``cmake_policy()`` command to set it to ``OLD`` or ``NEW``
explicitly.

.. note::
  The ``OLD`` behavior of a policy is
  ``deprecated by definition``
  and may be removed in a future version of CMake.

I guess we stick to legacy?