* configure.in: Add call to ACX_WITH_SYSTEM_VLIB.

* Makefile.am: Macro substitution for "zlib" subdir.
(Original patch from Anthon Pang <apang@telus.net>.)

* TODO (230): Add note about possible improvements to
ACX_WITH_EXTERNAL_ZLIB.
* aclocal.m4: Regenerated
* config.h.in: Ditto
* configure: Ditto
* Makefile.in: Ditto