From https://cmake.org/cmake/help/latest/module/FindOpenGL.html:
```
Linux-specific¶
Some Linux systems utilize GLVND as a new ABI for OpenGL. GLVND separates context libraries from OpenGL itself; OpenGL lives in "libOpenGL", and contexts are defined in "libGLX" or "libEGL". GLVND is currently the only way to get OpenGL 3+ functionality via EGL in a manner portable across vendors. Projects may use GLVND explicitly with target OpenGL::OpenGL and either OpenGL::GLX or OpenGL::EGL.
Projects may use the OpenGL::GL target (or OPENGL_LIBRARIES variable) to use legacy GL interfaces. These will use the legacy GL library located by OPENGL_gl_LIBRARY, if available. If OPENGL_gl_LIBRARY is empty or not found and GLVND is available, the OpenGL::GL target will use GLVND OpenGL::OpenGL and OpenGL::GLX (and the OPENGL_LIBRARIES variable will use the corresponding libraries). Thus, for non-EGL-based Linux targets, the OpenGL::GL target is most portable.
A OpenGL_GL_PREFERENCE variable may be set to specify the preferred way to provide legacy GL interfaces in case multiple choices are available. The value may be one of:
GLVND
If the GLVND OpenGL and GLX libraries are available, prefer them. This forces OPENGL_gl_LIBRARY to be empty.
Changed in version 3.11: This is the default, unless policy CMP0072 is set to OLD and no components are requeted (since components correspond to GLVND libraries).
LEGACY
Prefer to use the legacy libGL library, if available.
For EGL targets the client must rely on GLVND support on the user's system. Linking should use the OpenGL::OpenGL OpenGL::EGL targets. Using GLES* libraries is theoretically possible in place of OpenGL::OpenGL, but this module does not currently support that; contributions welcome.
OPENGL_egl_LIBRARY and OPENGL_EGL_INCLUDE_DIRS are defined in the case of GLVND. For non-GLVND Linux and other systems these are left undefined.
```
CMake 3.22.1 does not correctly interpret the conditional generator expression inside target_link_libraries when the library list is multiline. I could have put everything on one line but I preferred to treat the list externally to be able to manage it better.
Updated the README with instructions to successfully compile with Ubuntu.
- The minimum CMake version required is 3.14 (for "FetchContent_MakeAvailable").
- Improved management of old QT5 and old FFmpeg libraries (allowing compilation even on old distributions).
Since punes requires only one file from xdelta, there no point to
configure it with main autotools project, so let's just configure only
required macros for it.
