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

["john at johnmaddock dot co.uk" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #1 from John Maddock <john at johnmaddock dot co.uk> ---
I think I've found the problem - make the nu parameter negative and you get the
correct answers.

These functions appear to use the same definition for ellint_3 as GSL uses
(same code?), but TR1 uses a different one which matches A&S: note the change
of sign in the nu parameter in the integral:

GSL has Pi(\phi,k,n) = \int_0^\phi dt 1/((1 + n \sin^2(t))\sqrt(1 - k^2
\sin^2(t)))

TR1 has Pi(\phi,k,n) = \int_0^\phi dt 1/((1 - n \sin^2(t))\sqrt(1 - k^2
\sin^2(t)))