- Revive defining DEBUG in configure when --enable-debug is used.

- Add #define GNUC_UNUSED __attribute__ ((unused)), to be used on debug
  functions to silent unused function compilation warnings.
- Fix printf format on a function under #ifdef DEBUG.
- Add a prototype for a function which is ifdef'ed DEBUG.
- "#if 0" debug code in word-utils.c that should not normally be enabled on --enable-debug.