Use the generator_built variable to list any files which are built by
the generator. This ensures they get automatically rebuilt.
Three files were missing from EXTRA_DIST, so they were not included in
the tarball, meaning you would have needed OCaml to build from the
tarball. You would have seen this when building:
cd .. && /builddir/build/BUILD/libguestfs-1.31.9/generator/generator
written gobject/docs/guestfs-title.sgml
written customize/customize-synopsis.pod
written customize/customize-options.pod
generated 476921 lines of code