Exit CMakeLists if the library has already been defined.

This is to prevent the re-use of this library in multiple projects.

CMakeLists.txt

@@ -1,4 +1,9 @@
 cmake_minimum_required(VERSION 2.7)
+
+if(TARGET ds)
+  return()
+endif(TARGET ds)
+
 project(libds)
 
 set_property(GLOBAL PROPERTY C_STANDARD 90)