[Bug libstdc++/66689] comp_ellint_3 and ellint_3 return garbage values

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66689

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thanks for the analysis.

I'd like to re-use the TR1 code for an implementation of ISO/IEC 29124:2010
(whether or not it also goes into C++17) so we should fix this first.