Experimentally enable per-thread locale emulation

This is set to end in 5.39.10, but will give us field experience in the
meantime.

locale.c

@@ -133,9 +133,11 @@
  *    expects.  The perl core has made those changes, so pure perl programs
  *    become thread-safe.  Well-behaved XS code also keeps things thread-safe,
  *    either by not using locale-dependent system calls, or by changing to use
- *    the wrapper macros.  This layer is not chosen if the platform has native
- *    thread-safe locale handling.  Also, currently perl must have been
- *    Configured with "-Accflags=-DEMULATE_THREAD_SAFE_LOCALES".
+ *    the wrapper macros.  This layer is chosen on threaded perls where the
+ *    platform doesn't have working native thread-safe locale handling, and
+ *    there is no manual override in Configure.  To turn if off add these to
+ *    the Configuration options:
+        -Accflags='-DNO_THREAD_SAFE_LOCALE'" -Accflags='-DNO_POSIX_2008_LOCALE'"
  *
  *    This implementation is based on the observation that the underlying
  *    locale matters only to relatively few libc calls, and only during their

makedef.pl

@@ -180,6 +180,9 @@ BEGIN
     {
         $define{USE_THREAD_SAFE_LOCALE} = 1;
     }
+    else {
+        $define{EMULATE_THREAD_SAFE_LOCALES} = 1;
+    }
 }
 
 if ($define{USE_POSIX_2008_LOCALE} && $define{HAS_QUERYLOCALE})
@@ -189,7 +192,8 @@ BEGIN
     # Don't need glibc only code from perl.h
 }
 
-if ($define{USE_POSIX_2008_LOCALE} && ! $define{USE_QUERYLOCALE})
+if (  ($define{USE_POSIX_2008_LOCALE} && ! $define{USE_QUERYLOCALE})
+    || $define{EMULATE_THREAD_SAFE_LOCALES})
 {
     $define{USE_PL_CURLOCALES} = 1;
 }

perl.h

@@ -1251,6 +1251,8 @@ typedef enum {
 #    if  defined(USE_POSIX_2008_LOCALE)                                     \
      || (defined(WIN32) && (defined(_MSC_VER) || (defined(_UCRT))))
 #      define USE_THREAD_SAFE_LOCALE
+#    else
+#      define EMULATE_THREAD_SAFE_LOCALES
 #    endif
 #  endif