Add support for 'make help' in subdirectories

Including help for directory-specific targets, such as 'make 1' in ghc

compiler/Makefile

@@ -26,3 +26,12 @@ include $(TOP)/mk/sub-makefile.mk
 3:
 	+$(TOPMAKE) stage=3 all_ghc_stage3 compiler_stage3_NO_BUILD_DEPS=YES OMIT_PHASE_1=YES OMIT_PHASE_2=YES OMIT_PHASE_3=YES
 
+.PHONY: extra-help
+help : extra-help
+extra-help :
+	@echo "  make 1"
+	@echo "  make 2"
+	@echo "  make 3"
+	@echo
+	@echo "     Build the stage 1, 2 or 3 GHC respectively, omitting dependencies"
+	@echo "     and initial phases for speed."

ghc/Makefile

@@ -26,3 +26,12 @@ include $(TOP)/mk/sub-makefile.mk
 3:
 	+$(TOPMAKE) stage=3 all_ghc_stage3 compiler_stage3_NO_BUILD_DEPS=YES
 
+.PHONY: extra-help
+help : extra-help
+extra-help :
+	@echo "  make 1"
+	@echo "  make 2"
+	@echo "  make 3"
+	@echo
+	@echo "     Build the stage 1, 2 or 3 GHC respectively, omitting dependencies"
+	@echo "     and initial phases for speed."

libraries/Makefile

@@ -5,3 +5,11 @@ include $(TOP)/mk/sub-makefile.mk
 .PHONY: 1
 1 :
 	+$(TOPMAKE) stage1_libs
+
+.PHONY: extra-help
+help : extra-help
+extra-help :
+	@echo "  make 1"
+	@echo
+	@echo "     Build all libraries that are built by the stage1 GHC"
+	@echo

mk/sub-makefile.mk

@@ -30,7 +30,16 @@ STD_TARGETS = all clean distclean maintainer_clean install html ps pdf
 $(STD_TARGETS): 
 	+$(TOPMAKE) $@_$(dir)
 
-OTHERTARGETS=$(filter-out fast $(STD_TARGETS) $(SPEC_TARGETS),$(MAKECMDGOALS))
+OTHERTARGETS=$(filter-out fast help $(STD_TARGETS) $(SPEC_TARGETS),$(MAKECMDGOALS))
 .PHONY: $(OTHERTARGETS)
 $(OTHERTARGETS):
 	+$(TOPMAKE) $(dir)/$@
+
+.PHONY: help
+help : sub-help
+
+.PHONY: sub-help
+sub-help :
+	@echo "You are in subdirectory \"$(dir)\"."
+	@echo "Useful targets in this directory:"
+	@cat $(TOP)/SUBMAKEHELP