Use reentrant interface of qhull (2015+) when available

[woodychow]

New code should be written with libqhull_r. Existing users of libqhull should consider converting to libqhull_r. Although libqhull will be supported indefinitely, improvements may not be implemented. Reentrant qhull is 1-2% slower than non-reentrant qhull.
http://www.qhull.org/html/qh-code.htm

[taketwo]

Duplicate of #1565?

[SergioRAgostinho]

Definitely. @woodychow it would be great if you then could also review the current undergoing work at #1565 and contribute with your inputs there.

In the meantime I'll be closing this as a duplicate.

[woodychow]

Great. I did not know there is already a merge request. I did not have time to investigate the C++ interface; that's why I just quickly changed the code to use the re-entrant C interface for my stuff. I will take a look at #1565. Thanks.