Replace recursive calls to `make` with `$(MAKE)`

Ref: https://www.gnu.org/software/make/manual/make.html#Recursion

Ignores `deps/Makefile`. Hopefully addresses
https://github.com/cloudozer/ling/pull/10#issuecomment-67733795
Merge pull request #11 from lucafavatella/refactor-make-recursive-calls

Replace recursive calls to `make` with `$(MAKE)`
Express dependency between core and bc targets

```
$ git grep '\.\./' | grep '^[A-z]\+\/[A-z]\+:'
...
core/Makefile:117:include/bif.h: ../bc/scripts/bif.tab
core/Makefile:129:gentab/exp_tab.erl: $(PREMODS) ../bc/scripts/bif.tab
core/Makefile:130:      scripts/exptab_gen preload ../bc/scripts/bif.tab $@
...
```