OpenBSD doesn't define _POSIX_TIMERS because we are missing some of the
required clocks, but we do have enough for mpv.

Index: osdep/timer-linux.c
--- osdep/timer-linux.c.orig
+++ osdep/timer-linux.c
@@ -19,6 +19,8 @@
  */
 
 #include <unistd.h>
+#undef _POSIX_TIMERS
+#define _POSIX_TIMERS 1
 #include <stdlib.h>
 #include <time.h>
 #include <sys/time.h>