introducing WEBP_USE_THREAD switch in #46 breaks nostd build #47

libwebp-sys: v0.14.3

Hi!

I'm using libwebp-sys in a nostd binary (with Arm Cortex-M) and after a pull request #46 the library no longer builds, complaining about missing PTHREADS, which is a not a big surprise.

Could the fix introduced at the latest update be gated behind a "std" feature?

The thread issue is of no concern in a situation like I have.