Reworked the handling of "./configure --with-zlib=FOO", yielding several benefits:

  *  FOO can now point to both a flat directory as before or to a
	 directory one level up with lib/ and include/ subdirs, as zlib
	 installs to when configured with a --prefix, fixing an issue
	 | reported on the forum].
  *  The old method would check the default paths before trying
	 what you gave it, preventing local overrides when you know
	 the platform version isn't what you want and you've built from
	 source, then pointed Fossil at the replacement.  It isn't always safe
	 or advisable to remove the platform version in this case merely to
	 get Fossil's configuration script to move on and obey your explicit
	 directive.
  *  The old method checked for the header and the library in a single
	 step, resulting in unhelpful configure output, requiring that one
	 dig through config.log to figure out what happened.