Keep using undocumented gcc locale feature

[khwilliamson]

Since 5.39.7, this feature has been enabled experimentally by default. It is similar to querylocale(), found on *BSD systems. That function returns the current locale, and was created because this ability is often required, and POSIX neglected to define anything for this purpose. I'm told that the next POSIX Standard update will include a function for this purpose.

I have been using the gcc feature successfully for years in my local work spaces; we started this experiment to see if it worked generally. And there have been zero reports of problems with it.

The experiment was designed to refuse to compile in 5.39.10, so as to make sure it didn't slip into 5.40 without it being explicitly decided to do so. Now, the final steps in publishing 5.39.9 include bumping the release to 5.39.10, and those can't be done with the code as-is.

This commit declares the experiment a success, and enables this feature non-experimentally as the default, with the ability to turn it off by passing -Accflags=-DNO_USE_NL_LOCALE_NAME to Configure.

[leonerd]

OK - this switches the sense of the default and the arugment itself, but it doesn't remove the version-conditional #error line:

https://github.com/perl/perl5/commit/6eb3b0c7c89d29c37927b53a5142f06f8bd1621e#diff-f7d64c37bffc437cd59ec53833a08ce5adec3e7b03436c7ed8cb11c789b26ac0R395

[khwilliamson]

Oops; fixed