There are separate flags DBUG_OFF for disabling the DBUG facility
and ENABLED_DEBUG_SYNC for enabling the DEBUG_SYNC facility.
Let us allow debug builds without DEBUG_SYNC.
Note: For CMAKE_BUILD_TYPE=Debug, CMakeLists.txt will continue to
define ENABLED_DEBUG_SYNC.
When online alter rollbacks due to MDL time out, it doesn't mark the
index online status as ONLINE_INDEX_ABORTED. Concurrent update fails
to update the secondary index while building the entry.
InnoDB should check the online status of the secondary index before
building the secondary index entry.
Reviewed-by: Marko Mäkelä
