old versions of gcc, such as 4.2.1 which I'm in the process of adding
support for, have broken configure scripts that do not accept
variables on the command line and instead interpret them as the target
name. the easiest solution is just passing these in the environment.
previously the stage0 gcc was installed to the output location during
the build, then replaced later by the final gcc. this violates the
principles of make and simply wastes time. the stage0 gcc is usable
in-place without installing it, so we can simply do that instead.
Rich Felker