Add a new option to configure, --enable-minimal-install.
When this option is the only library grade that will be installed is the one
that is being used to build the compiler.  (This grade will also be set as the
default grade for the compiler that is being built.)  The intended use of this
option is to allow for the installation of a boostrap system from the source
distribution.

configure.ac:
    Implement --enable-minimal-install.

    Re-order some stuff.

README.boostrap:
    Users building a bootstrap compiler should use the new option
    rather than just doing --enable-libgrades=asm_fast.gc, since
    the latter won't necessarily result in a working system.

.INSTALL.in:
    Document the new option.