psql: Output dir and dependency generation for sql_help

psql: Output dir and dependency generation for sql_help

This is in preparation for building postgres with meson / ninja.

When building with meson, commands are run at the root of the build tree. Add
an option to put build output into the appropriate place. This can be utilized
by src/tools/msvc/ for a minor simplification, which also provides some
coverage for the new option.

To deal with dependencies to the variable set of input files to this script,
add an option to generate a dependency file (which meson / ninja can consume).

Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/5e216522-ba3c-f0e6-7f97-5276d0270029@enterprisedb.com