Failed to compile 7.3.1 under Sol7 - can't find libz.so (I think)

I'm having trouble compiling pg7.3.1 for Sol7.

The configure command I used was:
./configure --prefix=/usr/local/pgsql731 --enable-multibyte --with-java
--enable-syslog --without-readline

I get the following error:
ld.so.1: ./conftest: fatal: libz.so: open failed: No such file or directory

(I am compiling without readline 'cause it wouldn't compile with it,
even after installing it!)

Does anybody have a suggestion?

Thanks!

--
Matt