ext/POSIX/POSIX.xs: Recent commit triggers build-time warnings

[jkeenan]

Building perl on FreeBSD-13 with clang-14 as the compiler at v5.41.0-58-g21017bd9c1 I got these previously unseen build-time warnings:

  {
    char   => 6,
    group  => "Wunused-but-set-variable",
    line   => 5077,
    source => "POSIX.c",
    text   => "variable 'wday' set but not used",
  },
  {
    char   => 6,
    group  => "Wunused-but-set-variable",
    line   => 5078,
    source => "POSIX.c",
    text   => "variable 'yday' set but not used",
  },
  {
    char   => 6,
    group  => "Wunused-but-set-variable",
    line   => 5079,
    source => "POSIX.c",
    text   => "variable 'isdst' set but not used",
  },

I didn't formally bisect, but I suspect these were triggered by this recent change:

commit 86a9c18b6fab1949a26de790418b8b897a71e4ac
Author:     Karl Williamson <[email protected]>
AuthorDate: Sun Jun 16 10:17:11 2024 -0600
Commit:     Karl Williamson <[email protected]>
CommitDate: Wed Jun 19 19:50:39 2024 -0600

    Fix up and make sv_strftime_ints() public
    
    This enhanced function now allows you to specify if you want the system
    to consider the possibility of daylight savings time being in effect.
    Formerly, it was never considered.  As a result, the function is good
    enough to be made public.

@khwilliamson, can you take a look? Thanks.