Don't link with libz when not needed

Commit 8cc8e513633a1a8b12c416e32fb5362fcf4d65dd added check for libz
resulting in linking with lib z for all libraries. This is not expected
behavior. Patch solves it by making defining automake conditional so
cpghum is linked only if libz is available and LIBS variable is not
modified at all.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>