Fixed crash when setting query_cache_type to 0.

client/Makefile.am:
  Added zlib include (needed by checksum.c)
sql/set_var.cc:
  Updated call to disable_query_cache()
sql/sql_cache.cc:
  Don't give warning if we start mysqld with --query_cache_type=0 --query_cache-size=0
  Fixed crash when setting query_cache_type to 0 (we shouldn't call query_cache.disable_query_cache() when there is no current_thd)
sql/sql_cache.h:
  Added THD to disable_query_cache()