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)`