updated cmake file to avoid a warning from cmake

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402417

dlib/CMakeLists.txt

@@ -5,6 +5,7 @@
 
 # setting this makes CMake allow normal looking IF ELSE statements
 SET(CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS true)
+cmake_minimum_required(VERSION 2.4)
 
 # make macros that can add #define directives to the entire project.  Not just 
 # to the dlib library itself.  I.e. to dlib and to any projects that depend