* Descriptors now have a "system" field specifying the platform that

the build or run action should be perfomed on.  This ensures that
  descriptors have different hashes on different platforms.

src/Makefile

@@ -1,2 +1,13 @@
+all: nix nix-instantiate
+
+SYSTEM = $(shell ./config.guess)
+
 nix: nix.cc
-	g++ -g -Wall -o nix nix.cc -ldb_cxx-4
+	g++ -g -Wall -o nix nix.cc -ldb_cxx-4 -DSYSTEM=\"$(SYSTEM)\"
+
+nix-instantiate: nix-instantiate.in
+	sed "s/@SYSTEM@/$(SYSTEM)/" < $^ > $@
+	chmod +x $@
+
+clean:
+	rm -f *.o nix nix-instantiate