Don't use the deprecated pthread_yield() function

Per https://man7.org/linux/man-pages/man3/pthread_yield.3.html
       This call is nonstandard, but present on several other systems.
       Use the standardized sched_yield(2) instead.

Should fix #299.

src/xpdev/genwrap.h

@@ -304,8 +304,6 @@ DLLEXPORT int get_errno(void);
 	#if defined(XPDEV_THREAD_SAFE)
 		#if defined(__FreeBSD__)
 			#define MAYBE_YIELD()			pthread_yield()
-		#elif defined(_PTH_PTHREAD_H_)
-			#define MAYBE_YIELD()			pth_yield(NULL)
 		#elif defined(_POSIX_PRIORITY_SCHEDULING)
 			#define MAYBE_YIELD()			sched_yield()
 		#else