Make it optional for a kernel adapter to privde a map of preprocessor defines

[Eyal Rozenberg]

The preprocessor defines required to compile a kernel, or having potential effect on compiling a kernel, are not necessary information the kernel runner must be aware of in order to invoke the kernel: If the user sets the appropriate preprocessor definitions, compilation will succeed ; if the user doesn't, it will fail. This information is mostly useful for telling the user apriori which preprocessor definitions they can or must provide.

So, let's make it optional for a kernel adapter class to implement this method, and instead, have the base class return an indication of "unknown" (as opposed to an empty map, which is a valid return value on its own).