After having build PgPool-II on Red Hat Enterprise Linux 5.5 without any problem, I tried to build it on a fresh Debian Etch. The catch is that I did not want to install PostgreSQL 9.0, but just extract it from the binary packages provided by Entreprisedb (with option --extract-only 1).

Whatever options I passed to ./configure, it resulted in the same error:

checking for PQexecPrepared in -lpq... no
configure: error: libpq is not installed or libpq is old

Here is the answer: the binary package contains the libpq with the name libcrypto.so.0.9.8 (the RHEL name) when pgpool is looking libcrypto.so.6 on Debian. The same applies to libssl. So a simple

ln -s libcrypto.so.0.9.8 libcrypto.so.0.9.8
ln -s libssl.so.0.9.8 libssl.so.6

before your ./configure will solve it!