POSIX math: HP-UX support varies depending on HP cc.
POSIX math: NV_NAN can be the right answer for nan().
POSIX math: implement lround().

Somehow had missed this so far, but probably because in Perl terms
it is very similar to round(), and very similar to rint().

The API variations are fascinating: round(), roundl(), lround(), lroundl(),
llround(), llroundl().