changes required to build with cmake 3.22.2 on linux

[mpictor]

I'm not submitting a PR, as I doubt these changes are sufficiently backward compatible for merge as-is, and it's unlikely that I'd find the time to follow up on feedback etc etc. My hope is that someone else will find this useful and perhaps can massage it into a form suitable for merge.

The changes I made are:

1. set gl preference so cmake doesn't complain; not strictly necessary, and both GLVND and LEGACY seem to work.
2. in all CMakeLists.txt, replace GLUT_LIBRARY with GLUT_LIBRARIES

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d8525ba..58202ad 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -32,6 +32,8 @@
 
 cmake_minimum_required(VERSION 2.8.12)
 
+set(OpenGL_GL_PREFERENCE "GLVND")
+
 ######################################################################################
 # Project declaration and options
 ######################################################################################
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 8c6733f..6f1ffd1 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -25,11 +25,11 @@ if (Threads_FOUND AND OPENGL_FOUND AND GLUT_FOUND)
   add_executable(freenect-chunkview chunkview.c)
   add_executable(freenect-micview micview.c)
 
-  target_link_libraries(freenect-glview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
-  target_link_libraries(freenect-regview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
-  target_link_libraries(freenect-hiview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
-  target_link_libraries(freenect-chunkview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
-  target_link_libraries(freenect-micview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
+  target_link_libraries(freenect-glview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
+  target_link_libraries(freenect-regview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
+  target_link_libraries(freenect-hiview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
+  target_link_libraries(freenect-chunkview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
+  target_link_libraries(freenect-micview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
 
   install(TARGETS freenect-glview freenect-regview freenect-hiview freenect-chunkview freenect-micview
           DESTINATION bin)
@@ -51,7 +51,7 @@ if (BUILD_C_SYNC)
   if (OPENGL_FOUND AND GLUT_FOUND)
     include_directories(${OPENGL_INCLUDE_DIR} ${GLUT_INCLUDE_DIR})
     add_executable(freenect-glpclview glpclview.c)
-    target_link_libraries(freenect-glpclview freenect_sync ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${MATH_LIB})
+    target_link_libraries(freenect-glpclview freenect_sync ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${MATH_LIB})
     install(TARGETS freenect-glpclview
             DESTINATION bin)
   endif ()
diff --git a/wrappers/cpp/CMakeLists.txt b/wrappers/cpp/CMakeLists.txt
index 6d17924..c2222a8 100644
--- a/wrappers/cpp/CMakeLists.txt
+++ b/wrappers/cpp/CMakeLists.txt
@@ -18,8 +18,8 @@ if (BUILD_EXAMPLES)
     target_compile_features(freenect-cppview PUBLIC cxx_std_11)
     target_compile_features(freenect-cpp_pcview PUBLIC cxx_std_11)
 
-    target_link_libraries(freenect-cppview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
-    target_link_libraries(freenect-cpp_pcview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
+    target_link_libraries(freenect-cppview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
+    target_link_libraries(freenect-cpp_pcview freenect ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MATH_LIB})
 
     install(TARGETS freenect-cppview freenect-cpp_pcview
             DESTINATION bin)