Small fixes.

* [COWSPEAK,DEB] Added unit test for cowspeak.
* [DCG_ASCII,SC] Removed superfluous module imports.
* [DCG_GENERIC,SC] Removed superfluous module imports.
* [DCG_GENERIC,SC] Removed superfluous meta_predicate/1 directive.
* [RUN_EXT,EXT] Allow modules to register external program requirements.
* [TTS_EXT,EXT] Write a debug message if eSpeak is not installed.