Solaris install - "cannot compute sizeof (off_t)" error - readline issue?

I'm trying to install postgres 8.4.5 on Solaris 10 (Sun):
./configure --prefix=/app/postgres-8.4.5 \
--with-libs=/app/usr/local/lib \
--with-includes=/app/usr/local/include

(lib & include specified because I have readline installed in a
non-standard location. Yay.)

This fails with this error:
checking size of off_t... configure: error: cannot compute sizeof (off_t)

The config.log contains this (I'm not sure how much to include here):
conftest.c:71: warning: left shift count >= width of type
conftest.c:71: warning: left shift count >= width of type
conftest.c:73: error: size of array `off_t_is_large' is negative

...and this interesting tidbit, which I just discovered:
ld.so.1: conftest: fatal: libreadline.so.5: open failed: No such file
or directory

Per alvherre's suggestion on irc, I tried this:
./configure --prefix=/app/postgres-8.4.5 \
--with-libs=/app/usr/local/lib \
--with-includes=/app/usr/local/include \
--disable-largefile

It failed with the same command-line error message as above.

config.log contains the same error about readline:
ld.so.1: conftest: fatal: libreadline.so.5: open failed: No such file
or directory

The only seemingly-relevant thing I found on google was this:
http://www.xinotes.org/notes/note/747/
I don't have the right privs on this server to try that solution, so I
tried running configure --without-readline JFK, and that was
successful.

But I really want readline. ;) Any ideas what the problem is?

Thanks!

gabrielle