Makefile: little refactoring

And remove some warnings about openalut
```
src/notify.c: In function ‘play_sound_internal’:
src/notify.c:252:5: warning: ‘alutLoadWAVFile’ is deprecated (declared at /usr/include/AL/alut.h:113) [-Wdeprecated-declarations]
src/notify.c:258:5: warning: ‘alutUnloadWAV’ is deprecated (declared at /usr/include/AL/alut.h:116) [-Wdeprecated-declarations]
```

NOTE: since I never used openalut, I kept the old code as a comment.