* For efficiency: md5 integrated into nix.

* Command `nix ensure' which is like `nix getpkg' except that if the
  has refers to a run action it will just ensure that the imports are
  there.
* Command `nix closure' to print out the closure of the set of
  descriptors under the import relation, starting at a set of roots.
  This can be used for garbage collection (e.g., given a list of
  `activated' packages, we can delete all packages not reachable from
  those).
* Command `nix graph' to print out a Dot graph of the dependency
  graph.
* `nix-addroot' adds a root for the (unimplemented) garbage collector.

src/Makefile

@@ -2,8 +2,16 @@ all: nix nix-instantiate
 
 SYSTEM = $(shell ./config.guess)
 
-nix: nix.cc
-	g++ -g -Wall -o nix nix.cc -ldb_cxx-4 -DSYSTEM=\"$(SYSTEM)\"
+nix: nix.o md5.o
+	g++ -g -o $@ $^ -ldb_cxx-4
+
+%.o: %.cc
+	g++ -g -Wall -o $@ -c $< -DSYSTEM=\"$(SYSTEM)\"
+
+%.o: %.c
+	gcc -g -Wall -o $@ -c $< -DSYSTEM=\"$(SYSTEM)\"
+
+md5.o: md5.c md5.h
 
 nix-instantiate: nix-instantiate.in
 	sed "s/@SYSTEM@/$(SYSTEM)/" < $^ > $@