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: svn://svn.transmissionbt.com/Transmission/trunk@14322 f4695dd4-2c0a-0410-b89c-da849a56a58e
use pkg-config to get the zlib cflags/libs; remove obsolete zlib.m4 macro

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14323 f4695dd4-2c0a-0410-b89c-da849a56a58e