Use of C99 extra long double math functions after r236148

[Stephen Montgomery-Smith]

This function seems to be able to compute clog with a worst case 
relative error of 4 or 5 ULP.

For clogf I simply used double precision.  That seems to get a errors as 
high as 1000 ULP.  It might be simpler just to have clogf use clog:

float complex clogf(float complex z) {
   return (clog(z));
}