#4 Automatically download source archives

[Tom Pike]

Checking out the foldingcd svn repository can take several hours due to the source archives being checked into the repository. This patch will automatically download the source archives so they do not need to be included in the repository (checking out from our location is significantly slower than fetching the sources directly). It also adds two extra phony targets:

* prefetch: to download all source archives.
* distclean: to remove all source archives automatically.

[Tom Pike]

[Tom Pike]

This patch requires the two previous patches #3115445 & #3115434.