COMP: support for cmake < 2.6.4

CMake/PreventInSourceBuilds.cmake

 #
 # This function will prevent in-source builds
 function(AssureOutOfSourceBuilds)
-  # make sure the user doesn't play dirty with symlinks
-  get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
-  get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+  if (${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION} GREATER 2.6)
+    # make sure the user doesn't play dirty with symlinks
+    get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
+    get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+  else()
+    get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" ABSOLUTE)
+    get_filename_component(bindir "${CMAKE_BINARY_DIR}" ABSOLUTE)
+  endif()
 
   # disallow in-source builds
   if("${srcdir}" STREQUAL "${bindir}")