dtk-generic say conflicts with macOS say

### Steps to reproduce

Use `spd-say` on macOS

### Obtained behavior

Incorrectly uses macOS [`say`](https://manp.gs/mac/1/say) as DECTalk `say` causing a file name to be spoken.

Presumably the `GenericCmdDependency` can be used to verify if this is DECTalk say.

### Expected behavior

Uses dtk-generic output module only if `say` binary is from DECTalk.

Even better, have a generic macOS `say` module and use that if it's available :D