linking error

I am running Redhat linux (2.0.34) and postgreSQL 6.3.2.  I'm trying to
use the C-interface via libpq, but I'm having linking problems.  The
following is the result when attempting to compile sample1 from the
postgres libpq documentation:

[rbarrett@localhost rbarrett]$ gcc -O3 -Wall sample1.c -I
/usr/local/pgsql/include/ -L /usr/local/pgsql/lib/ -lpq

/usr/local/pgsql/lib//libpq.a(fe-auth.o): In function
`pg_password_sendauth':
fe-auth.o(.text+0x17): undefined reference to `crypt'

Does anyone know how I can fix this?  Need I upgrade to 6.4?

Thanks,
    Randy Barrett