#218 zutil.h is not a public header file

[Darren Salt]

zutil.h, according to its documentation, is an internal
zlib header file; it is not present in zlib1g-dev
(Debian), and it has not merely been omitted from that
package - "make install" (in the zlib source) does not
install it.

vba must *NOT* rely on zutil.h.