Don't report error if position is not set

Cython/Compiler/Errors.py

@@ -176,7 +176,7 @@ def message(position, message, level=1):
 def warning(position, message, level=0):
     if level < LEVEL:
         return
-    if Options.warning_errors:
+    if Options.warning_errors and position:
         return error(position, message)
     warn = CompileWarning(position, message)
     line = "warning: %s\n" % warn