[Bug libstdc++/83140] assoc_legendre returns negated value when m is odd

[emsr at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83140

--- Comment #2 from emsr at gcc dot gnu.org ---
The fact that Boost followed us into this makes the situation interesting. We
are the only two impls that I know of. I like the std convention slightly
better but maybe we should ask for a lib change.

or not.