slib build-time dependency

Hello,

While I maintain Gauche package in Debian, I realize that slib
dependency blocks unstable -> testing migration.  See:

    https://tracker.debian.org/pkg/gauche

Basically, the problem is: which package is responsible for slibcat?

While it's mainly an issue of Debian packaging (of gauche and slib), I
think that it could be handled by Gauche itself cleanly, so that a user
can use gauche + slib consistently under a distribution packaging
system.

Currently, slib is detected by configure at build time, and it's a kind
of *hard* one.  slibcat is generated by Gauche at build time of Gauche.

FYI, in Debian, slibcat is provided by slib package for other Scheme
implementations, like:

	slib: /var/lib/slib/gambc/slibcat
	slib: /var/lib/slib/mitscheme/slibcat
        slib: /var/lib/slib/chez/slibcat

In the case of Gauche, it's

	gauche: /usr/share/gauche-0.98/0.9.15-p2/lib/slibcat

Possibly, we can remove the dependency by gauche; Then, slib package
will be not needed at build time of Gauche.  We could only specify
SLIB_DIR=/var/lib/slib/gauche and let slib package provide slibcat for
gauche.

This means that it will be slib package which will depends on gauche
package (to generate slibcat at build time of slib or installation time
of slib).  This allows upgrading slib package.  And for gauche package,
slib is optional.

Just an idea, not tested at all.  How do you think?
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