Update CMakeLists.txt

CMakeLists.txt

@@ -227,6 +227,7 @@ INCLUDE_DIRECTORIES( BEFORE
   include
   ${CMAKE_CURRENT_BINARY_DIR}
   ${CMAKE_CURRENT_BINARY_DIR}/include
+  contrib/
 )
 
 LIST(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake-modules" )