move source tarballs out of the scm

hi!

since the external source tarballs (e.g. boost/download/*) add their
size to the repository each time a new version is committed, it's one of
the major sources of the growth of the repository.

the cws ause110 implements a different approach to deal with these large
binary files:

- a file named ooo.lst lists the tarballs required for that specific
source version with the first line being the URL where to find them

- the sh script fetch_tarballs.sh knows how to fetch these files via
http and places them in a directory TARFILE_LOCATION, by default
"ext_sources" beside the repository

- when building, dmake knows where to get them by looking at the
environment variable TARFILE_LOCATION

if not disabled by the configure switch "--disable-fetch-external", the
bootstrap step updates TARFILE_LOCATION according to ooo.lst.
using "--with-external-tar" sets TARFILE_LOCATION to a place of own choice.

that's the idea, comments welcome...


tschau...

ause