CMake: update policy to 3.11.

In particular this enables linking with GLVND on Linux by default.

CMakeLists.txt

@@ -8,7 +8,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
 endif()
 
 cmake_minimum_required(VERSION 3.2.0 FATAL_ERROR)
-cmake_policy(VERSION 3.2.0)
+cmake_policy(VERSION 3.11)
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH}
     "${CMAKE_SOURCE_DIR}/cmake/")
 set(CMAKE_CXX_STANDARD 11)