auto merge of rust-lang#108 : jdeseno/cargo/clean-submodules, r=alexcrichton

Small change. Use git's `submodule` `foreach` command to cleanup dependencies. I think this way you won't have to remember to update the makefile when a dependency is added or removed.

Author: bors

Makefile

@@ -83,9 +83,7 @@ clean:
 	rm -rf target
 
 clean-all: clean
-	cd libs/hamcrest-rust && make clean
-	cd libs/hammer.rs && make clean
-	cd libs/toml-rs && make clean
+	git submodule foreach make clean
 
 install:
 	install -d $(DESTDIR)$(BINDIR)