qhull: darwin compatibility

The hand-rolled iterator declaration is incompatible with libc++'s, and
clang > 3.5 is stricter about this.

See
<https://lists.freebsd.org/pipermail/freebsd-ports-bugs/2014-December/298242.html>
for another reference to this issue.

The errors that occur are of the form:
reference to 'random_access_iterator_tag' is ambiguous