Best way to confine project to 64 bit builds only?

[NonNull]

I have inherited an open source C project that assumes that the 
size of a long and the size of a pointer are the same, and I have 
translated it into very similar D just like 
https://dlang.org/blog/2018/06/11/dasbetterc-converting-make-c-to-d/

D has the size of long fixed at 64 bits, so a pointer now has to 
be 64 bits. So I want to put something into the source to ensure 
an attempt to make a 32 bit build fails. What is the best way to 
do this?