Improve `Depends` specs in debian package. (#5675)

This is basically a contrived way of adding a `Recommends` on `libpq5`, to fix #5653.

The way this is supposed to happen in debhelper is to run
`dh_shlibdeps`, which in turn runs `dpkg-shlibdeps`, which spits things out
into `debian/<package>.substvars` whence they can later be included by
`control`.

Previously, we had disabled `dh_shlibdeps`, mostly because `dpkg-shlibdeps`
gets confused about PIL's interdependent objects, but that's not really the
right thing to do and there is another way to work around that.

Since we don't always use postgres, we don't necessarily want a hard Depends on
libpq5, so I've actually ended up adding an explicit invocation of
`dpkg-shlibdeps` for `psycopg2`.

I've also updated the build-depends list for the package, which was missing a
couple of entries.

1 files changed, 5 insertions, 3 deletions

diff --git a/docs/postgres.rst b/docs/postgres.rst
index 0ae52ccbd8..e08a5116b9 100644
--- a/docs/postgres.rst
+++ b/docs/postgres.rst
@@ -11,7 +11,9 @@ a postgres database.
 
 * If you are using the `matrix.org debian/ubuntu
   packages <../INSTALL.md#matrixorg-packages>`_,
-  the necessary libraries will already be installed.
+  the necessary python library will already be installed, but you will need to
+  ensure the low-level postgres library is installed, which you can do with
+  ``apt install libpq5``.
 
 * For other pre-built packages, please consult the documentation from the
   relevant package.
@@ -34,7 +36,7 @@ Assuming your PostgreSQL database user is called ``postgres``, create a user
    su - postgres
    createuser --pwprompt synapse_user
 
-Before you can authenticate with the ``synapse_user``, you must create a 
+Before you can authenticate with the ``synapse_user``, you must create a
 database that it can access. To create a database, first connect to the database
 with your database user::
 
@@ -53,7 +55,7 @@ and then run::
 This would create an appropriate database named ``synapse`` owned by the
 ``synapse_user`` user (which must already have been created as above).
 
-Note that the PostgreSQL database *must* have the correct encoding set (as 
+Note that the PostgreSQL database *must* have the correct encoding set (as
 shown above), otherwise it will not be able to store UTF8 strings.
 
 You may need to enable password authentication so ``synapse_user`` can connect