interface/cpp.cc: store C++ callback function in a struct
interface/cpp.cc: store C++ callback function in a struct

Store the C++ callback function in a struct for passing
to the C callback function.  This avoids the need for
a (naked) double indirection on the C++ callback function and
should be less error-prone because the type of the callback
function is now checked by the compiler.
The use of a struct also allows additional pieces
of data to be passed to the C callback function.
This will be exploited in the next commit.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl,PollyLabs/isl,PollyLabs/isl