- 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.
Merge pull request #3 from ampli/ifdefdebug

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