The name "hash.c" is already used in db/hash/hash.c, and having duplicated
names, aside from it being bad style, breaks the tools/nbcompat build
because it picks the wrong hash.c file. Thanks to greg for helping debug
this.
when the strings get expanded we gain an _ from namespace.h. So define a
macro to point to the appropriate include file for the hash function, and
include the hash function later, from the .c file we always include. This
allows us to eliminate the hack of including namespace.h twice as well as
nbtool_config.h.
