Makefile: fix recursive invocation to use `$(MAKE)`

Calling `make` is known-bad in many cases, but never as much as here:

```
make[2]: Nothing to be done for `pre-all'.
touch theories/algebra/ofe.vo
touch theories/algebra/monoid.vo
touch theories/algebra/cmra.vo
[...]
touch theories/heap_lang/lib/array.vo
make[2]: Nothing to be done for `post-all'.
make[1]: *** wait: No child processes.  Stop.
gmake: *** [Makefile:3: all] Error 2
```