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.