Tweak the shell scripting for descending into and building subdirs.

Tweak the shell scripting for descending into and building subdirs.

In general, use this:
	@for dir in $(SUBDIRS) ; do \
		if [ -d $$dir ] ; then \
			(cd $$dir && $(MAKE)) || exit 1; \
		fi \
	done

Basically, silently skip missing subdirs but generate an error and stop if
there's a compilation or install problem.
This was done inconsistantly before.  In once case, a missing subdir was
causing us to go into an infinte loop!