Add basic make {help,help-hidden} targets.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@93713 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile b/Makefile
index 82efba1..7941f10 100644
--- a/Makefile
+++ b/Makefile
@@ -24,6 +24,27 @@
 ###
 # Top level targets
 
+# FIXME: Document the available subtargets.
+help:
+	@echo "usage: make [{VARIABLE=VALUE}*] target"
+	@echo
+	@echo "User variables:"
+	@echo "  VERBOSE=1: Use to show all commands [default=0]"
+	@echo
+	@echo "Available targets:"
+	@echo "  clean: clean all configurations"
+	@echo "  test:  run unit tests"
+	@echo "  all:   build all configurations"
+	@echo
+
+help-hidden: help
+	@echo "Debugging variables:"
+	@echo "  DEBUGMAKE=1: enable some Makefile logging [default=0]"
+	@echo
+	@echo "Debugging targets:"
+	@echo "  make-print-FOO: print information on the variable 'FOO'"
+	@echo
+
 # Provide default clean target which is extended by other templates.
 .PHONY: clean
 clean::