cmake: Link libGLX when X11 support is requested

[marv]

Using cmake (version 3.23.0) the build fails to link the utils

[...]
[ 87%] Linking C executable bin/visualinfo                                                                                                                      [113/5249]
/usr/x86_64-pc-linux-gnu/bin/cmake -E cmake_link_script CMakeFiles/visualinfo.dir/link.txt --verbose=1
/usr/host/bin/x86_64-pc-linux-gnu-cc -Wall -g -ggdb3 -O0 -rdynamic CMakeFiles/visualinfo.dir/home/marv/scm/github/glew/src/visualinfo.c.o -o bin/visualinfo  -Wl,-rpath,/h
ome/marv/scm/github/glew/build-obj/lib: lib/libGLEW.so.2.2.0 -lSM -lICE -lX11 -lXext -lOpenGL -lSM -lICE -lX11 -lXext
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: CMakeFiles/visualinfo.dir/home/marv/scm/github/glew/src/visualinfo.c.o: in function `main':
/home/marv/scm/github/glew/src/visualinfo.c:198: undefined reference to `glXQueryExtensionsString'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: CMakeFiles/visualinfo.dir/home/marv/scm/github/glew/src/visualinfo.c.o: in function `CreateContext':
/home/marv/scm/github/glew/src/visualinfo.c:1204: undefined reference to `glXQueryExtension'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: /home/marv/scm/github/glew/src/visualinfo.c:1206: undefined reference to `glXChooseVisual'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: /home/marv/scm/github/glew/src/visualinfo.c:1209: undefined reference to `glXCreateContext'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: /home/marv/scm/github/glew/src/visualinfo.c:1221: undefined reference to `glXMakeCurrent'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: CMakeFiles/visualinfo.dir/home/marv/scm/github/glew/src/visualinfo.c.o: in function `DestroyContext':
/home/marv/scm/github/glew/src/visualinfo.c:1227: undefined reference to `glXDestroyContext'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: lib/libGLEW.so.2.2.0: undefined reference to `glXGetProcAddressARB'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: lib/libGLEW.so.2.2.0: undefined reference to `glXQueryVersion'
/usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-ld: lib/libGLEW.so.2.2.0: undefined reference to `glXGetClientString'
collect2: error: ld returned 1 exit status

Commit 2b50f4a ("CMake: Prefer GLVND if available") changed the cmake policy CMP0072 to NEW in order to make the FindOpenGL module prefer the GLVND libraries (libOpenGL and libGLX) over the legacy GL library (libGL).

The help of the CMP0072 policy states:

CMP0072

.. versionadded:: 3.11

FindOpenGL prefers GLVND by default when available.

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 3.23.0 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.

The changes from the mentioned commit combined with the new behaviour of the FindOpenGL module resulted in the glew library being linked against libOpenGL instead of libGL, but not libGLX. Since libOpenGL doesn't link against GLX compared to libGL the linker errors above surfaced. Fix this by explicitly linking against libGLX if GLEW_X11 is enabled

[nigels-com]

Looks good.