OpenBSD build fix. still no cpu affinity.

- enabling pthread_get/pthread_set_name_np api.
- disabling per thread cpu affinity handling, unsupported on this platform.

src/background_thread.c

@@ -80,6 +80,9 @@ background_thread_info_init(tsdn_t *tsdn, background_thread_info_t *info) {
 
 static inline bool
 set_current_thread_affinity(int cpu) {
+#ifdef __OpenBSD__
+	return false;
+#else
 #if defined(JEMALLOC_HAVE_SCHED_SETAFFINITY)
 	cpu_set_t cpuset;
 #else
@@ -110,6 +113,7 @@ set_current_thread_affinity(int cpu) {
 #  endif
 	return ret != 0;
 #endif
+#endif
 }
 
 #define BILLION UINT64_C(1000000000)