Disable the new compiler warning about ambiguous exception return values when...

Disable the new compiler warning about ambiguous exception return values when the function is external (and therefore possibly not under the control of the user).

Cython/Compiler/Nodes.py

@@ -758,7 +758,9 @@ class CFuncDeclaratorNode(CDeclaratorNode):
                     if not return_type.assignable_from(self.exception_value.type):
                         error(self.exception_value.pos,
                               "Exception value incompatible with function return type")
-                    if (return_type.is_int or return_type.is_float) and self.exception_value.has_constant_result():
+                    if (visibility != 'extern'
+                            and (return_type.is_int or return_type.is_float)
+                            and self.exception_value.has_constant_result()):
                         try:
                             type_default_value = float(return_type.default_value)
                         except ValueError: