* SGML_DOC can now hold a _list_ of sgml documentations to build
* both building.sgml and ffi-art.sgml are now build in docs/

*MERGE*

$(SGML_DOC) can only support one document, so build the Build System
Guide by default.

Added docbook-cheat-sheet

"installing" is now called "building"

clean up document makefiles.

SGML version of the installation guide.  SGML-Tools 1.0.3 or later is required,
and the indexing still doesn't work automatically.

More small changes towards 2.02

Major update to more-or-less 2.02