since we might use them in the future
See: libvips/libvips#1443.
We had exception specs on the C++ interface, but C++11 does not support them and some compilers have never honoured them. Remove all specs. Thanks Lovell. See https://github.com/jcupitt/libvips/issues/362
to cplusplus, see https://github.com/jcupitt/libvips/issues/230