locale.c: Use strerror_l if platform has it

strerror_l makes the my_strerror function trivial, as it doesn't have to
worry about critical sections, etc.  Even on unthreaded perls, it avoids
having to change the current locale, and then change it back.

Author: khwilliamson

locale.c

@@ -2728,9 +2728,27 @@ Perl_my_strerror(pTHX_ const int errnum)
 
     const bool within_locale_scope = IN_LC(LC_MESSAGES);
 
-#  ifdef USE_POSIX_2008_LOCALE
+#  if defined(HAS_POSIX_2008_LOCALE) && defined(HAS_STRERROR_L)
+
+    /* This function is trivial if we have strerror_l() */
+
+    if (within_locale_scope) {
+        errstr = strerror(errnum);
+    }
+    else {
+        errstr = strerror_l(errnum, PL_C_locale_obj);
+    }
+
+    errstr = savepv(errstr);
+
+#  else /* Doesn't have strerror_l(). */
+
+#    ifdef USE_POSIX_2008_LOCALE
+
     locale_t save_locale = NULL;
-#  else
+
+#    else
+
     char * save_locale = NULL;
     bool locale_is_C = FALSE;
 
@@ -2739,7 +2757,7 @@ Perl_my_strerror(pTHX_ const int errnum)
      * setlocale() ) */
     LOCALE_LOCK;
 
-#  endif
+#    endif
 
     DEBUG_Lv(PerlIO_printf(Perl_debug_log,
                             "my_strerror called with errnum %d\n", errnum));
@@ -2761,7 +2779,7 @@ Perl_my_strerror(pTHX_ const int errnum)
                                     "uselocale returned 0x%p\n", save_locale));
         }
 
-#  else    /* Not thread-safe build */
+#    else    /* Not thread-safe build */
 
         save_locale = setlocale(LC_MESSAGES, NULL);
         if (! save_locale) {
@@ -2781,7 +2799,7 @@ Perl_my_strerror(pTHX_ const int errnum)
             }
         }
 
-#  endif
+#    endif
 
     }   /* end of ! within_locale_scope */
     else {
@@ -2807,7 +2825,7 @@ Perl_my_strerror(pTHX_ const int errnum)
         }
     }
 
-#  else
+#    else
 
         if (save_locale && ! locale_is_C) {
             if (! setlocale(LC_MESSAGES, save_locale)) {
@@ -2820,7 +2838,8 @@ Perl_my_strerror(pTHX_ const int errnum)
 
     LOCALE_UNLOCK;
 
-#  endif
+#    endif
+#  endif /* End of doesn't have strerror_l */
 #endif   /* End of does have locale messages */
 
 #ifdef DEBUGGING