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.

Per input from @mlkersten.