build: implement autotools build system for libopusfile. (v4)

Includes
- A make debug target that disables optimizations and enables
   assertions,
- Proper ./configure switches for the optional features,
- A configuration summary,
- libtool versioning information,
- Visibility and warning flags,
- API documentation, and
- Support for out-of-tree builds.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>