These two patches speed up library building:
Do the "runtime" directory before doing the platform directory, because
platform drivers need a file from the runtime directory (the old code
ran "runtime/Makefile" three times for each target!!!).
Clean only the shared directories -- the target directories don't need
cleaning.
Changes by Greg King.
git-svn-id: svn://svn.cc65.org/cc65/trunk@5019 b7a2c559-68d2-44c3-8de9-860c34a00d81