Fix CMake 3.11 warning about FindOpenGL.

cmake/libigl.cmake

@@ -323,10 +323,19 @@ endif()
 ### Compile the opengl part ###
 if(LIBIGL_WITH_OPENGL)
   # OpenGL module
-  find_package(OpenGL REQUIRED)
   compile_igl_module("opengl")
-  target_link_libraries(igl_opengl ${IGL_SCOPE} ${OPENGL_gl_LIBRARY})
-  target_include_directories(igl_opengl SYSTEM ${IGL_SCOPE} ${OPENGL_INCLUDE_DIR})
+
+  # OpenGL library
+  if (NOT CMAKE_VERSION VERSION_LESS "3.11")
+    cmake_policy(SET CMP0072 NEW)
+  endif()
+  find_package(OpenGL REQUIRED)
+  if(TARGET OpenGL::GL)
+    target_link_libraries(igl_opengl ${IGL_SCOPE} OpenGL::GL)
+  else()
+    target_link_libraries(igl_opengl ${IGL_SCOPE} ${OPENGL_gl_LIBRARY})
+    target_include_directories(igl_opengl SYSTEM ${IGL_SCOPE} ${OPENGL_INCLUDE_DIR})
+  endif()
 
   # glad module
   if(NOT TARGET glad)