Checking whether a precondition is strengthening a super-defined precondition seems to be to strict in case of using external contract classes.

[timmr]

Given the following scenario

1. Having a SuperClass with external contract class SuperContractClass and methode void method(int a)
2. TargetClass, a sub class of SuperClass, with external contract TargetContractClass
3. no if (precondition()) block inside the definition of method method of SuperContractClass
4. but a if (precondition()) inside the overrided method method in TargetContractClass
5. setting the configuration variable <default-precondition> to `undefined

` (or omitting it)

Expected behaviour: no "Found strengthening pre-condition" error

Observed behaviour: "Found strengthening pre-condition in method(int)" error is raised