Removes dependency on pthread library and uses fast Slim Reader/Writer (SRW) locks for NSLock/NSRecursiveLock/NSCondition/NSConditionLock as well as all internal locks. Adds GS_MUTEX_*() macros in GSPThread.h, that are being used for all internal locking instead of pthread APIs.
Also adds support for thread priorities on Windows, fixes method signature of +[NSThread setThreadPriority:] to match Apple platforms, and adds error handling in same method.
