Replace
.PRECIOUS: %.hs
with
.SECONDARY: %.hs
.PRECIOUS is the wrong thing: it keeps the file when 'make' is
killed, as well as preventing automatic deletion. .SECONDARY just
prevents the automatic deletion.
Spotted by: Ian Lynagh.
#-----------------------------------------------------------------------------
# Happy Suffix Rules
#
-.PRECIOUS: %.hs
+.SECONDARY: %.hs
%.hs : %.ly
$(HAPPY) $(HAPPY_OPTS) $<