Don't use pthread on OpenBSD

since OpenBSD pthread does not support pthread_condattr_setpshared().
This patch could also stay in the OpenBSD ports tree or depend on a
configure test macro rather than defined __OpenBSD__.

include/shmlog.h

@@ -11,7 +11,9 @@
 #pragma once
 
 #include <stdint.h>
+#if !defined(__OpenBSD__)
 #include <pthread.h>
+#endif
 
 /* Default shmlog size if not set by user. */
 extern const int default_shmlog_size;
@@ -37,8 +39,10 @@ typedef struct i3_shmlog_header {
      * and don’t matter — clients use an equality check (==). */
     uint32_t wrap_count;
 
+#if !defined(__OpenBSD__)
     /* pthread condvar which will be broadcasted whenever there is a new
      * message in the log. i3-dump-log uses this to implement -f (follow, like
      * tail -f) in an efficient way. */
     pthread_cond_t condvar;
+#endif
 } i3_shmlog_header;