[opus] [libopusfile PATCH] build: implement autotools build system for libopusfile. (v2)

[Timothy B. Terriberry]

Diego Elio Pettenò wrote:
> This time it's complete with assertions on make debug, proper
> ./configure switches for the optional features, visibility and warning
> flags, and summary at the end of the configuration.

Also:
../configure: line 11818: syntax error near unexpected token `default,'
../configure: line 11818: `CC_ATTRIBUTE_VISIBILITY(default,'