Instead of trying to make weak functions work on all platforms,

just use a registration function for a plugin-specific setlocale
function.  The sudoers version just wraps sudoers_setlocale().

plugins/sudoers/sudoers.c

@@ -228,6 +228,8 @@ sudoers_policy_main(int argc, char * const argv[], int pwflag, char *env_add[],
     volatile int rval = true;
     debug_decl(sudoers_policy_main, SUDOERS_DEBUG_PLUGIN)
 
+    sudo_warn_set_locale_func(sudoers_warn_setlocale);
+
     /* Is root even allowed to run sudo? */
     if (user_uid == 0 && !def_root_sudo) {
 	/* Not an audit event. */
@@ -553,6 +555,8 @@ done:
     sudo_endpwent();
     sudo_endgrent();
 
+    sudo_warn_set_locale_func(NULL);
+
     debug_return_int(rval);
 }