subjects.mk: $(tsrc) info/findsubj.pl sources.mk
perl $(srcdir)/info/findsubj.pl
+# If sources.mk is out of date, $(tsrc) is. If it contains files that
+# went away, make can't remake subjects.mk. Tell it to ignore such
+# missing files:
+$(tsrc):
$(tsubj): info/mksubj.pl
perl $(srcdir)/info/mksubj.pl $@ $(filter %.t, $^)