Simplification of `autogen.sh'

[Ludovic Courtès]

Hi,

In the process of preparing 1.8.2, I added into `guile-core' files that
were initially in `workbook/build/dist-files' and removed `BUGS' from
the distribution files (this bug report mechanism had been essentially
unused since 2003).  While I was at it, I removed from `autogen.sh' code
that relied on `workbook' and copied the aforementioned files from
there.

The rationale is to make `guile-core' self-contained and thereby to
simplify the release process.

I believe the motivation behind `workbook/dist-files' was to avoid
duplicating files across packages that were all released by the Guile
team.  Since `guile-core' is currently the only package left (AFAICS),
it seemed worthwhile to adopt a more standard layout and make it
self-contained.

Thanks,
Ludovic.