Remove config.cache files when reconfiguring at top level

[Alfred M. Szmidt]

   >> 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.

   That's the only way to override cross-compile defaults in target
   libraries.

Can't you pass the variables on the command line?  I.e.

.../configure ac_cv_foo=123

I've used this for overriding cross-compile defaults in various
programs though never had the need for gcc/gdb/binutils.