Bump to the *real* minimum required version. Setting CMP0022 breaks CMake versions < 2.8.12

cmake/CMakeLists.txt

@@ -1,5 +1,5 @@
 # Minimum CMake required
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.12)
 
 # Project
 project(protobuf C CXX)