MXS-2315: Use BRE with std::regex

The default ECMAScript syntax appears to be broken on CentOS 7 which
effectively prevents its use in most cases. A more reliable alternative
would be to use the bundled PCRE2 library but the basic POSIX regular
expressions seem to work.

server/modules/monitor/csmon/csmon.cc

@@ -56,8 +56,10 @@ static std::string do_query(MXS_MONITORED_SERVER* srv, const char* query)
 // Returns a numeric version similar to mysql_get_server_version
 int get_cs_version(MXS_MONITORED_SERVER* srv)
 {
+    // GCC 4.8 appears to have a broken std::regex_constants::ECMAScript that doesn't support brackets
+    std::regex re("Columnstore \\([0-9]*\\)[.]\\([0-9]*\\)[.]\\([0-9]*\\)-[0-9]*",
+                  std::regex_constants::basic);
     std::string result = do_query(srv, "SELECT @@version_comment");
-    std::regex re("Columnstore ([0-9]*)[.]([0-9]*)[.]([0-9]*)-[0-9]*");
     std::smatch match;
     int rval = 0;