this allows sharing a sources directory between multiple checkouts or
independent toolchain-using projects.
also, during distclean, only remove the in-tree sources directory, not
a custom/external one. this is a safety measure in case the SOURCES
variable is set to something stupid like blank, root directory, or
some large shared directory.
this makes it easy to rebuild components simply by removing the source
or build directory with rm -rf; no separate action on the steps files
is needed.
some dependencies are also changed to order-only so that
update/rebuild of one component does not force unnecessary rebuild of
another.
new convenience pseudo-targets are also added.
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