Hi,

with Luis' commit of today the trunk is no longer able to
be compiled with gcc-4.x.

The problem is std::is_trivially_default_constructible is
not defined before gcc-5 although the compiler supports C++11 

I am not sure about what's the best approach for conditionally
enabling the code, especially for compilers other than g++.


This fixes the build for me.
Is it OK for trunk?


Thanks
Bernd.