Use of C99 extra long double math functions after r236148

[Stephen Montgomery-Smith]

I did a ULP test on clog.  The test code is attached.  (Not the cleanest 
code, I know, but it does the job.)  It needs the mpfr and unuran ports 
installed.

To my shock, I found that under certain circumstances, the ULP in the 
real part was huge.  The problem is when hypot(x,y) is close to 1, 
because then the real part of clog is close to zero.  I was seeing ULPs 
in the thousands.

I struggled to find a solution, and now I think I have the ULP down to 
about 2.  I am going to work on it more tomorrow to see if I can get ULP 
down even further.