tolerate old pcre versions without PCRE2_EXTENDED_MORE flag

mysql-test/main/func_regexp_pcre.result

@@ -819,8 +819,11 @@ SET default_regex_flags=DEFAULT;
 SELECT 'Aq' RLIKE 'A\\q';
 ERROR 42000: Regex error 'unrecognized character follows \ at offset 2'
 SET default_regex_flags='EXTRA';
-SELECT 'Aq' RLIKE 'A\\q';
-ERROR 42000: Regex error 'unrecognized character follows \ at offset 2'
+SELECT 'A' RLIKE 'B';
+'A' RLIKE 'B'
+0
+Warnings:
+Warning	1105	PCRE2 doesn't support the EXTRA flag. Ignored.
 SET default_regex_flags=DEFAULT;
 SELECT 'a\nb\nc' RLIKE '^b$';
 'a\nb\nc' RLIKE '^b$'

mysql-test/main/func_regexp_pcre.test

@@ -384,9 +384,10 @@ SET default_regex_flags=DEFAULT;
 
 --error ER_REGEXP_ERROR
 SELECT 'Aq' RLIKE 'A\\q';
+
 SET default_regex_flags='EXTRA';
---error ER_REGEXP_ERROR
-SELECT 'Aq' RLIKE 'A\\q';
+SELECT 'A' RLIKE 'B';
+
 SET default_regex_flags=DEFAULT;
 
 SELECT 'a\nb\nc' RLIKE '^b$';

sql/sys_vars.cc

@@ -5992,6 +5992,11 @@ static Sys_var_set Sys_log_disabled_statements(
        DEFAULT(LOG_DISABLE_SP),
        NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(check_has_super));
 
+#define NOT_SUPPORTED_YET -2
+#ifndef PCRE2_EXTENDED_MORE
+#define PCRE2_EXTENDED_MORE NOT_SUPPORTED_YET
+#endif
+
 static const char *default_regex_flags_names[]= 
 {
   "DOTALL",    // (?s)  . matches anything including NL
@@ -6024,9 +6029,11 @@ int default_regex_flags_pcre(THD *thd)
     {
       if (default_regex_flags_to_pcre[i] < 0)
       {
+        const char *msg= default_regex_flags_to_pcre[i] == NOT_SUPPORTED_YET
+          ? "Your version of PCRE2 does not support the %s flag. Ignored."
+          : "PCRE2 doesn't support the %s flag. Ignored.";
         push_warning_printf(thd, Sql_condition::WARN_LEVEL_WARN,
-                            ER_UNKNOWN_ERROR,
-                            "PCRE2 doens't support the EXTRA flag. Ignored.");
+                          ER_UNKNOWN_ERROR, msg, default_regex_flags_names[i]);
         continue;
       }
       res|= default_regex_flags_to_pcre[i];