Resolve Squeeze installation problems with PostgreSQL 9.0

Resolve Squeeze installation problems with PostgreSQL 9.0

When you install OpenSRF on Debian Squeeze, one of the packages
(apache2-prefork-dev) installs 8.4 versions of libpq5 and libpq-dev as
dependencies. These conflict with the 9.0 versions that are required as
dependencies for PostgreSQL 9.0 - so get them out of the way, pin the
9.0 versions of the packages, and reinstall apache2-prefork-dev.

We will probably need to do something similar for Lucid.

Signed-off-by: Dan Scott <dan@coffeecode.net>