Use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR

so that it works when we are included by another project.

CMakeLists.txt

@@ -43,7 +43,7 @@ enable_testing()
 # Macro to add files to a var. Can even be used in subdirectories.
 # Source: http://stackoverflow.com/a/7049380/194894
 macro (add_files var)
-    file (RELATIVE_PATH _relPath "${CMAKE_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
+    file (RELATIVE_PATH _relPath "${PROJECT_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
     foreach (_src ${ARGN})
         if (_relPath)
             list (APPEND ${var} "${_relPath}/${_src}")