Add a counter for the number of select statements using window functions

The counter is available via SHOW [GLOBAL] STATUS and will be reported
by the feedback plugin.

sql/mysqld.cc

@@ -8411,6 +8411,7 @@ SHOW_VAR status_vars[]= {
   {"Feature_subquery",         (char*) offsetof(STATUS_VAR, feature_subquery), SHOW_LONG_STATUS},
   {"Feature_timezone",         (char*) offsetof(STATUS_VAR, feature_timezone), SHOW_LONG_STATUS},
   {"Feature_trigger",          (char*) offsetof(STATUS_VAR, feature_trigger), SHOW_LONG_STATUS},
+  {"Feature_window_functions", (char*) offsetof(STATUS_VAR, feature_window_functions), SHOW_LONG_STATUS},
   {"Feature_xml",              (char*) offsetof(STATUS_VAR, feature_xml), SHOW_LONG_STATUS},
   {"Flush_commands",           (char*) &show_flush_commands, SHOW_SIMPLE_FUNC},
   {"Handler_commit",           (char*) offsetof(STATUS_VAR, ha_commit_count), SHOW_LONG_STATUS},

sql/sql_class.h

@@ -789,6 +789,7 @@ typedef struct system_status_var
   ulong feature_timezone;	    /* +1 when XPATH is used */
   ulong feature_trigger;	    /* +1 opening a table with triggers */
   ulong feature_xml;		    /* +1 when XPATH is used */
+  ulong feature_window_functions;   /* +1 when window functions are used */
 
   /* From MASTER_GTID_WAIT usage */
   ulonglong master_gtid_wait_timeouts;          /* Number of timeouts */

sql/sql_select.cc

@@ -2699,6 +2699,8 @@ bool JOIN::make_aggr_tables_info()
     if (curr_tab->window_funcs_step->setup(thd, &select_lex->window_funcs,
                                            curr_tab))
       DBUG_RETURN(true);
+    /* Count that we're using window functions. */
+    status_var_increment(thd->status_var.feature_window_functions);
   }
 
   fields= curr_fields_list;