Log Message:
Support user-destdir.

Generate catalog file in ${DESTDIR}, then package it.  This should
work, because the catalog file (${PREFIX}/share/guile/1.8/slibcat)
is only provided for slib to know what interpreter is using slib;
i.e., the file is written once at installation.

(Let's see if this theory is correct.  If not, we'll revisit this.)

Reviewed By:	gdt (maintainer)