Changes from previous version are to define $(gcc_version), and some
variables defined based on it, directly in the Makefiles instead of as
autoconf substitutions; and to use := for every variable defined to a
$(shell) expression, which avoids reading them more than once and may
also fix the mysterious serialization of bootstrap (if true, this
would be a bug in Make, although fortunately one with an easy
workaround).
I still don't have very high confidence in this, but I am getting
close to feeling that it is better than the status quo. I'm doing
another test cycle myself, and if it succeeds and the installation
looks correct, I will probably check this in. More testing would be
appreciated, especially with --enable-version-specific-runtime-libs.
Comments are greatly appreciated, too.