Add test for --dep-info.

src/test/run-make/dep-info/Makefile

+-include ../tools.mk
+all:
+	$(RUSTC) --dep-info --lib lib.rs
+	sleep 1
+	touch foo.rs
+	-rm -f $(TMPDIR)/done
+	$(MAKE) -f Makefile.foo
+	rm $(TMPDIR)/done
+	pwd
+	$(MAKE) -df Makefile.foo
+	rm $(TMPDIR)/done && exit 1 || exit 0

src/test/run-make/dep-info/Makefile.foo

+ifeq ($(shell uname),Darwin)
+LIBEXT=dylib
+else
+LIBEXT=so
+endif
+
+$(TMPDIR)/libfoo-b517899a-0.1.$(LIBEXT):
+	$(RUSTC) --dep-info --lib lib.rs
+	touch $(TMPDIR)/done
+
+-include $(TMPDIR)/lib.d

src/test/run-make/dep-info/bar.rs

+pub fn bar() {}

src/test/run-make/dep-info/foo.rs

+pub fn foo() {}

src/test/run-make/dep-info/lib.rs

+#[pkgid="foo#0.1"];
+
+pub mod foo;
+pub mod bar;