Display an error when building DLLs that do not export any functions

On Windows, building a DLL that does not export any functions will not generate an import library or exports file. If anything tries to link to that DLL, the compiler will error out, unable to find the .lib.

It would be helpful if QBS analyzed built DLL artifacts to see whether they export any functions, and if they do not, display an warning message like "mydll.dll does not export any functions. Did you forget to decorate classes with __declspec(dllexport)?"

I can't think of any use cases that warrant a DLL with no exported functions, but I'm sure there are some. However in most cases I think issuing a warning would be helpful.