Fix building with a shared zlib: ensure link with '-lz'.

Configuring for a shared zlib:

    LDFLAGS="-L/opt/local/lib -R/opt/local/lib" \
        ./configure --shared-zlib \
        --shared-zlib-includes=/opt/local/include \
        --shared-zlib-libpath=/opt/local/lib

and building would fail on SmartOS because the link would not include '-lz'.
This doesn't fail on Mac (at least in my setup) because you get lucky with the
openssl libs reported from the Homebrew pkg-config:

    $ pkg-config --libs openssl
    -lssl -lcrypto -lz

On SmartOS, the pkgsrc libs for openssl are:

    $ pkg-config --libs openssl
    -Wl,-R/opt/local/lib -L/opt/local/lib -lssl -lcrypto -lsocket -lnsl -ldl

The patch has no adverse effect on the Mac build (by control case).