Fix -std=c++98 -Wzero-length-array

This is another follow-up fix to
commit b393e2cb0c
which turned out to be still broken.

Replace the C++11 keyword 'constexpr' with #define.

debug_sync_t::str: Remove the zero-length array.
Replace sync->str with reinterpret_cast<char*>(&sync[1]).

storage/innobase/handler/ha_innodb.cc

@@ -19388,7 +19388,7 @@ innobase_debug_sync_callback(srv_slot_t *slot, const void *value)
 	// One allocatoin for list node object and value.
 	void *buf = ut_malloc_nokey(sizeof(srv_slot_t::debug_sync_t) + len);
 	srv_slot_t::debug_sync_t *sync = new(buf) srv_slot_t::debug_sync_t();
-	strcpy(sync->str, value_str);
+	strcpy(reinterpret_cast<char*>(&sync[1]), value_str);
 
 	rw_lock_x_lock(&slot->debug_sync_lock);
 	UT_LIST_ADD_LAST(slot->debug_sync, sync);

storage/innobase/include/srv0srv.h

@@ -615,7 +615,7 @@ extern ulong	srv_fatal_semaphore_wait_threshold;
 /** Buffer pool dump status frequence in percentages */
 extern ulong srv_buf_dump_status_frequency;
 
-constexpr uint srv_max_purge_threads = 32;
+#define srv_max_purge_threads 32
 
 # ifdef UNIV_PFS_THREAD
 /* Keys to register InnoDB threads with performance schema */
@@ -1132,12 +1132,9 @@ struct srv_slot_t{
 						(only used for user threads) */
 #ifdef UNIV_DEBUG
 	struct debug_sync_t {
-		UT_LIST_NODE_T(debug_sync_t)
-			debug_sync_list;
-		char str[0];
+		UT_LIST_NODE_T(debug_sync_t) debug_sync_list;
 	};
-	UT_LIST_BASE_NODE_T(debug_sync_t)
-		debug_sync;
+	UT_LIST_BASE_NODE_T(debug_sync_t) debug_sync;
 	rw_lock_t debug_sync_lock;
 #endif
 };

storage/innobase/row/row0purge.cc

@@ -1218,9 +1218,10 @@ row_purge_step(
 	while (UT_LIST_GET_LEN(slot->debug_sync)) {
 		srv_slot_t::debug_sync_t *sync =
 					UT_LIST_GET_FIRST(slot->debug_sync);
+		const char* sync_str = reinterpret_cast<char*>(&sync[1]);
 		bool result = debug_sync_set_action(current_thd,
-						    sync->str,
-						    strlen(sync->str));
+						    sync_str,
+						    strlen(sync_str));
 		ut_a(!result);
 
 		UT_LIST_REMOVE(slot->debug_sync, sync);

storage/innobase/srv/srv0srv.cc

@@ -663,14 +663,14 @@ char	srv_buffer_pool_dump_at_shutdown = TRUE;
 char	srv_buffer_pool_load_at_startup = TRUE;
 
 /** Slot index in the srv_sys.sys_threads array for the master thread. */
-constexpr ulint	SRV_MASTER_SLOT = 0;
+#define SRV_MASTER_SLOT 0
 
 /** Slot index in the srv_sys.sys_threads array for the purge thread. */
-constexpr ulint	SRV_PURGE_SLOT	= 1;
+#define SRV_PURGE_SLOT 1
 
 /** Slot index in the srv_sys.sys_threads array from which purge workers start.
   */
-constexpr ulint	SRV_WORKER_SLOTS_START = 2;
+#define SRV_WORKER_SLOTS_START 2
 
 #ifdef HAVE_PSI_STAGE_INTERFACE
 /** Performance schema stage event for monitoring ALTER TABLE progress