myisam/mi_extra.c:
  More debugging information
mysql-test/r/union.result:
  Added test for UNION bug
mysql-test/t/union.test:
  Added test for UNION bug
sql/sql_base.cc:
  Clear field->query_id when using UNION
sql/sql_parse.cc:
  Fix for UNION bug
sql/sql_union.cc:
  Cleanup
sql/table.h:
  Fix for UNION bug
tools/Makefile.am:
  Fix for compiling with MIT-threads