[opus] Symbol visibility control in static builds

[Alexey Izbyshev]

Hello,

Opus currently always sets default visibility for exported functions 
when OPUS_BUILD macro is set. However, it is not a desirable behavior 
when libopus is statically linked with the final binary (e.g. libxul.so 
in firefox), since all symbols get exported even with 
-fvisibility=hidden. In firefox, this problem is partially solved by 
wrapping translation units with #pragma visibility push(hidden), which 
is stronger than the command line flag, however, only libopus functions 
that are actually used in firefox code are affected, and others are 
still exported.

It would be great if Opus provided some macro (like OPUS_STATIC_BUILD) 
that, when set, would make OPUS_EXPORT empty even when OPUS_BUILD is 
set. Can it be provided in future versions?

--
Alexey Izbyshev