Don't generate constructor wrappers if a base class has a private constructor

g++-5 errors out with this now with errors such as:

default_constructor_wrap.cxx:665:27: error: use of deleted function ‘FFF::FFF()’
   result = (FFF *)new FFF();
                           ^
default_constructor_wrap.cxx:314:7: note: ‘FFF::FFF()’ is implicitly deleted because the default definition would be ill-formed:
 class FFF : public F {
       ^
default_constructor_wrap.cxx:301:4: error: ‘F::~F()’ is private
    ~F() { }
    ^
default_constructor_wrap.cxx:314:7: error: within this context

Examples/test-suite/ruby/default_constructor_runme.rb

@@ -143,9 +143,6 @@ end
 # This should work fine
 f = F.new
 
-# This should work fine
-ff = FFF.new
-
 # This should work fine
 g = G.new