Set kvazaar config threads to 0 if ENABLE_MULTITHREADING_SUPPORT is false

[meowtec]

The background:

I am trying to build libheif to wasm with kvazaar.
The building is successful but it run with error from kvazaar:

--threads=auto value set to 4.
pthread_create failed!
Could not initialize threadqueue.

It seems that kvazaar doesn't provide a build option for disable threads, the only way is settings config->threads to 0 at runtime.

[farindk]

Thanks