Experimentally enable glibc undocumented querylocale()

Experimentally enable glibc undocumented querylocale()

This is querylocale() by another name, and is undocumented, hence we
haven't enabled it by default.  But it seems to work fine.  In order to
gain wider experience in using it, it is here default-enabled through
5.39.9 (unless we decide to end the experiment earlier), at which point
a compilation error will remind us to decide to keep it or take it out.

I put the check in locale.c instead of the more obvious perl.h, because
the definition would come earlier in perl.h than the PERL_VERSION macros
are defined, and I don't think its worth moving things around for just a
potential of a few releases.