Warn about undefined local functions.

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2991

Reported by Martin Stjernholm mast@roxen.com

Consider:

    class A
    {
      local int a();
      void f() {werror ("%O\n", a());}
    }

    class B
    {
      inherit A;
      int a() {return 17;}
    }

    int main()
    {
      B()->f();
    }

The local modifier here doesn't have any effect; in 7.3 the call to a in f will be dynamically bound and the program prints "17" (in 7.2 it will fail with an obscure backtrace). I think a warning or perhaps even an error is in order in this case, instead of silently ignoring the local modifier. The same applies if local::a is used in the code somewhere in A.