I'm not a Makefile expert, but it looks like this rule will only rebuild the app if any file in its top level directory of the app is modified. For example it won't update it if a file "email/js/something.js" is modified.
Again, as I'm not an expert with makefile I don't know how to properly set such recursive sub folder dependencies. But google told me something like this should work:
```
APP_DEPS =$(shell find $(2) -print)
$(1): $(APP_DEPS) ...
```