Mac compile patch for AltMath

[michael-petersen]

I missed this in the recent merge of #101: the cylindrical Bessel function issue bit me again on Mac.

Also one piece of cruft can be removed.

[The9Cat]

Good find on dead.letter. It must be mine but I don't know how it got there!

The fix looks fine to me. Another option is to remove expontest.cc altogether as it is really a sanity check on the Hankel transform and has doesn't add any functionality to any EXP-based work flow.

[michael-petersen]

Yes, I was also wondering if expontest.cc could just be removed. It could potentially be helpful in a force-accuracy test for Cylinder as well, right? In which case it might make sense to keep it.

[The9Cat]

It is a standalone example for the PotRZ class (which makes a 3d gravitational potential from a 2d polar density) using the underlying QDHT Hankel transform class. So, yes, perhaps it's worth keeping as a usage case?

Okay, I'm convinced. Let's merge it.