standerdise on 3.1.0

CMakeLists.txt

@@ -24,16 +24,8 @@ if (USE_QT)
     set_property(CACHE DESIRED_QT_VERSION PROPERTY STRINGS 4 5)
 endif()
 
-if (APPLE)
-    # OS X build process relies on this fix: https://github.com/Kitware/CMake/commit/3df5147043d83aa09acd5c9ce31d5c602efb99db
-    cmake_minimum_required(VERSION 3.1.0)
-elseif (USE_QT AND DESIRED_QT_VERSION MATCHES 5)
-    # 2.8.11+ is required to make Qt5 happy and allow linking QtMain on Windows.
-    cmake_minimum_required(VERSION 2.8.11)
-else()
-    # We probably support older versions than this.
-    cmake_minimum_required(VERSION 2.6)
-endif()
+# set the minimum required version across the board
+cmake_minimum_required(VERSION 3.1.0)
 
 project(OpenMW)