A simple C thread pool implementation

Currently, the implementation: * Works with pthreads only, but API is intentionally opaque to allow other implementations (Windows for instance). * Starts all threads on creation of the thread pool. * Reserves one task for signaling the queue is full. * Stops and joins all worker threads on destroy.

Possible enhancements

The API contains addtional unused 'flags' parameters that would allow some additional options:

• Lazy creation of threads (easy)
• Reduce number of threads automatically (hard)
• Unlimited queue size (medium)
• Kill worker threads on destroy (hard, dangerous)
• Support Windows API (medium)
• Reduce locking contention (medium/hard)