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

Makefile

@@ -15,6 +15,9 @@ else
 	INSTALL_PREFIX="$(SDKROOT)"
 endif
 
+help::
+	echo Use make install DSTROOT=<destination>
+
 installsrc:: $(SRCROOT)
 
 	ditto $(SRCDIRS)/include $(SRCROOT)/include