Add missing comment in all Makefiles

The comment for the rule diff_alt_orig was missing. It was added like:

    sgit -e '/diff_alt_orig:/i \
    # diff alt and orig' '*Makefile'

2000/dlowe/Makefile

@@ -187,6 +187,7 @@ diff_orig_alt: ${PROG}.orig.c
 	    ${DIFF} -u ${PROG}.orig.c ${PROG}.alt.c; \
 	fi
 
+# diff alt and orig
 diff_alt_orig:
 	@-if [[ -e ${PROG}.alt.c ]]; then \
 	    ${DIFF} -u ${PROG}.alt.c ${PROG}.orig.c; \