- Add an @@optimizer_switch flag for table_elimination for debug build
- Better comments 

mysql-test/t/index_merge_myisam.test:
  MWL#17: Table elimination: last fixes
  - Add an @@optimizer_switch flag for table_elimination for debug build
sql/mysql_priv.h:
  MWL#17: Table elimination: last fixes
  - Add an @@optimizer_switch flag for table_elimination for debug build
sql/mysqld.cc:
  MWL#17: Table elimination: last fixes
  - Add an @@optimizer_switch flag for table_elimination for debug build