Remove config.cache files when reconfiguring at top level

[Nick Clifton]

Hi Andreas,

> This should be optional, as it also erases any manual changes added to
> the cache file.

Ah - I did not know that users would ever modify the cache files.  Given 
that they do though - is it right that they should expect their changes 
to be preserved across a reconfigure ?  I would have thought that the 
cache files would have the same status as Makefiles - OK to modify for 
temporary local changes but never expected to last for very long.

Cheers
   Nick