Consider enabling / exposing parallel feature of cc crate

[w-flo]

Hi, and thanks for maintaining these bindings!

Since libwebp takes quite long to compile, maybe the parallel feature of the cc crate would be useful for this crate? Or maybe it could be exposed in the libwebp-sys2 crate as a feature (like in libwebp-sys) to leave that choice up to the user of this crate.