Enable -Wno-error=c++11-narrowing for clang builds

...until underlying issues are fixed

Author: nyalldawson

CMakeLists.txt

@@ -330,7 +330,7 @@ IF ("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
   ENDIF()
 ELSEIF ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
   SET(USE_CXX_11 TRUE)
-  SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+  SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wno-error=c++11-narrowing")
 ELSE()
   SET(USE_CXX_11 FALSE)
 ENDIF()