does zlib really need to build multilib?

[Jack Howarth]

   I noticed that in gcc trunk and gcc 4.2 branch
that multilib builds of zlib occur. Does gcc
actually use the multlib zlib? For instance on
x86_64 linux does the 32-bit zlib get used or on
Darwin does the 64-bit zlib get used? We are
considering using the --with-system-zlib option
when building gcc 4.2 but MacOS X 10.4 and earlier
lacks a 64-bit zlib support library. Thanks in
advance for any clarifications.
           Jack