Fix function-defined-but-not-used warning.

Fix function-defined-but-not-used warning.

Buildfarm member jacana (MinGW) has been complaining that
get_iso_localename is defined but not used.  This is evidently
fallout from the recent removal of VS2013 support in pg_locale.c.
Rearrange the #ifs so that get_iso_localename and its subroutine
search_locale_enum won't get built on MinGW.

I also noticed that a comment in get_iso_localename cross-
referenced a comment in IsoLocaleName that isn't there anymore.
Put back what I think is the referenced material.