Looks like ancient versions of libpqxx (like the ones shipped on Ubuntu 18.04)
don't have exec_prepared(). So test for libpqxx 7 and conditionally use work::prepared() if we find an older version.

pqxx::connection::activate() and pqxx::connection::disconnect() are gone,
the former doesn't have a corresponding substitute. Instead the caller is
encouraged to watch out for broken_connection exception. Also the explicit
activation of a PostgreSQL connection is no longer supported.

work::prepared() is also no longer supported, so change this to
work::exec_prepared(). There's no need to special case this for older libpqxx
versions, since this method is already present there, too.

This commit wraps those deprecated API calls into a ifdef to support
builds with an older libpqxx version. We might get rid of this if we don't want
to have support for those older ones.

libpqxx 7 requires C++17 and thus requires us to tell the compiler to use
the right compiler flag. Teach autoconf to test for the proper switches
depending on the libpqxx version found.

This also defines a new macro HAVE_LIBPQXX7 in config.h, which will be
used by the follow up commit to reflect recent API changes in libpqxx 7.

Cf. https://www.postgresql.org/message-id/20210506162651.GJ27406@telsasoft.com

Newer distributions work without it, just add it unconditionally.

This appears to provoke stack overflows when testing MonetDB.

Cleaning it is a bad idea when building from a release tarball.

Reported-by: @alexanderlaw
Closes #19

filter.sql was replaced by known.txt and known_re.txt.

See https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=fd1a421fe66173fb9b85d3fe150afde8e812cbe4

Blind attempt at increasing generation speed.