Resuming downloads of external files

[Stephan Bergmann]

On 01/18/2016 11:16 PM, Chris Sherlock wrote:
> We’d just have to add in the "-C -“ switches to Curl.
>
> Are there any concerns in making this occur?

Note that, at least with --with-external-tar=..., it can happen that 
multiple builds try to download concurrently.  I don't know whether the 
current code handles that gracefully, nor whether adding -C would have 
an effect on that.