configuring in-tree gmp/mpfr with "none"?

[Jay]

When gcc configures the in-tree gmp/mpfr, why
does it use --host=none-${host_vendor}-${host_os} 
--target=none-${host_vendor}-${host_os} 

instead of  --host=${host_alias} --target=${target_alias}

This "breaks" config.cache if used across directories, because the platforms 
change.

I'm trying like:
  mkdir /obj  
  cd /obj  
  /src/gcc/configure -cache-file=`pwd`/config.cache  
  # must delete the initial one as well, else another problem 
  rem config.cache  
  make  

With the 4.3.1 release.

Thanks,
 - Jay