Unbreak the build of `lang/gauche' when option SLIB is set: by default,
upstream expects that SLIB catalogue is generated *after* everything is
installed. They are aware that this logic does not go along well with
staging (e.g. for RPM, they suggest it to be called in the postinstall
script), and also provide ``slibcat-in-place'' target which is exactly
what we need for staging.

Patch the ``Makefile.in'' since I find these changes easier to follow/
debug than changes to the port's Makefile suggested in the PR.

PR: 226653