MSVC only got stdbool.h in MSVC 2013; provide a manual typedef
for this compiler instead.
The C interface test makes sure that this doesn't break the ABI.
Add a struct that matches the C++ interface vtable.
This requires that the C++ interface methods are declared to use
the same calling convention as normal C functions, and that the
C struct exactly matches the layout and ordering of the C++
virtual table - MSVC seemed to reorder methods if there were
overloaded methods.
This is required to make the order in the C++ virtual table
consistent in MSVC - previously the overloaded methods were
ordered differently in the vtable compared to the interface
declaration.
