Add workaround for msvc-12 faulty array initialization.

[awulkiew]

This is a workaround for the following error:

boost/geometry/srs/projections/par_data.hpp(92) : error C2536: 'boost::geometry::srs::detail::towgs84<T>::boost::geometry::srs::detail::towgs84<T>::m_data' : cannot specify explicit initializer for arrays

I added core/config.hpp file allowing us to add defines conveniently.

[barendgehrels]

Thanks

[barendgehrels]

Cool, I've added this file too - for removing rescaling.
Your's will be merged earlier so I will adapt/extend it