How to "fool" apt-get into believing a package is installed?

[Neal Lippman <nl@lippman.org>]

I am looking to install squid onto a system which runs my imap mail
store, using cyrus-imap. As part of the cyrus installation, I have also
installed cyrus-sasl. I compiled both myself, because at the time (over
1 year ago) that I did this, the version of cyrus-sasl and cyrus-imap
running in stable and testing was quite old and outdated.

My problem now is that squid depends on libsasl2, which I already have
installed via my self-compiled cyrus-sasl. Rather than risk losing or
breaking my cyrus-imap installation by installing the debian libsasl2
package, it would be better to make apt- realize that I already have a
sasl2 library installed.

Is there a reasonable way to do this?

Thanks.
nl