Issue6: build fix for Python 3.4

This should fix issue #6.  There can be several possible ways to handle this, including:
- introduce a wrapper macro to the (non)existence of the trailing structure member (as this branch does)
- enable -Wno-missing-field-initializers for python wrappers
- require Python 3.4 or higher

None of these seems ideal to me, but I thought the first one is least bad in that we don't have to
weaken the compiler warning level and it keeps compatibility.