Expose feature for shared stdxx on android

[NiklasEi]

The cpal feature seems to be needed to get bevy + kira to work on my android phone.

[tesselode]

Is this always going to be needed on Android, or only when using Kira with Bevy? If it's the former, can we just make this a platform-specific dependency?

[NiklasEi]

I don't think it's always needed, so I would follow the way cpal and bevy expose this via a feature flag.

From the cpal readme:

Oboe can either use a shared or static runtime. The static runtime is used by default, but activating the oboe-shared-stdcxx feature makes it use the shared runtime, which requires libc++_shared.so from the Android NDK to be present during execution.

[NiklasEi]

FYI: the same solution has been taken in rodio now: RustAudio/rodio#546

[tesselode]

ok fiiiine i'll merge it

(thank you for your investigation!)