Pass external dependencies to submodule Makefiles

Part of the fix for #2137.
auto merge of #2472 : mbrubeck/servo/2137-done-deps, r=metajack

Part of the fix for #2137.  This allows submodules to correctly recompile (or not) based on all of the dependency information from `mk/sub.mk`.  Along with this change, we will also need to make changes to each of the affected submodules' make files, for example in `rust-azure/Makefile.in`:

```diff
-libazure.dummy: azure.rc $(RUST_SRC) libazure.a ../../skia/skia/libskia.a
+libazure.dummy: azure.rc $(RUST_SRC) libazure.a $(EXT_DEPS)
```

r? @metajack