Make the default Makefile less destructive:  John McCall

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@134375 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile b/Makefile
index 62d1bb3..14acf4c 100644
--- a/Makefile
+++ b/Makefile
@@ -15,6 +15,9 @@
 	INSTALL_PREFIX="$(SDKROOT)"
 endif
 
+help::
+	echo Use make install DSTROOT=<destination>
+
 installsrc:: $(SRCROOT)
 
 	ditto $(SRCDIRS)/include $(SRCROOT)/include