Very belatedly, many changes to smooth many rough edges with the C back

Very belatedly, many changes to smooth many rough edges with the C back
end.  It should now be possible to bootstrap and install the system
using the C back end much more cleanly, by the following steps:

  # Prepare the tree for compiling from C.
  ./Setup.sh
  ./configure --enable-native-code=c
  make c

  # Optionally, clean everything except the bare minimum of the C code
  # to generate a distribution tree.  Doing so requires reconfiguring.
  make c-clean
  # ...distribute the tree..
  ./configure --enable-native-code=c

  # Make the system from C, without needing an existing Scheme
  # installation for bootstrapping.
  make

  # Install.
  make install