Add output directory option to gen_node_support.pl

Add output directory option to gen_node_support.pl

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.

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