Fixed compiler warnings and warnings from build.tags

Other things
- Ensure that ut_d() is set to EXPR if ut_ad() is DEBUG_ASSERT()
  If not, we will get a crash in purge_sys_t::~purge_sys_t() as
  this ut_ad() code expect's that the ut_d() codes has been executed

sql/sql_admin.cc

@@ -394,7 +394,9 @@ static bool open_only_one_table(THD* thd, TABLE_LIST* table,
                  open_and_lock_tables(thd, table, TRUE, 0));
   }
 
+#ifndef DBUG_OFF
 dbug_err:
+#endif
 
   thd->prepare_derived_at_open= FALSE;