FindOpenGL: Default to non-GLVND libraries for legacy GL
Projects using OPENGL_LIBRARIES
or OpenGL::GL
expect legacy GL.
Although GLVND OpenGL+GLX provides legacy GL interfaces, using those
library files may conflict with legacy GL library files used by
dependencies (or dependents) of such projects. Therefore we should
not use OpenGL+GLX when a legacy GL library is available.
If OPENGL_gl_LIBRARY
is set then use it as the legacy GL library.
If it is not set then fall back to using GLVND OpenGL+GLX to provide
legacy GL interface. This will allow users to build projects using
GLVND even if they have not been ported.
Fixes: #17437 (closed)