since libz's been required in rpcimpl.c for ages and nobody's complained, remove the HAVE_ZLIB cruft for conditional-compiling with and without zlib.

git-svn-id: file:///data/workdir/Transmission-svn//trunk@14322 1bcf1a7e-6170-4b50-b784-84ec52b2dec6
use pkg-config to get the zlib cflags/libs; remove obsolete zlib.m4 macro

git-svn-id: file:///data/workdir/Transmission-svn//trunk@14323 1bcf1a7e-6170-4b50-b784-84ec52b2dec6