fixed OpenBSD support (#921)

Index: src/utilities.cc
--- src/utilities.cc.orig
+++ src/utilities.cc
@@ -293,6 +293,8 @@ pid_t GetTID() {
   return getpid();  // Linux:  getpid returns thread ID when gettid is absent
 #elif defined GLOG_OS_WINDOWS && !defined GLOG_OS_CYGWIN
   return static_cast<pid_t>(GetCurrentThreadId());
+#elif defined GLOG_OS_OPENBSD
+  return getthrid();
 #elif defined(HAVE_PTHREAD)
   // If none of the techniques above worked, we use pthread_self().
   return (pid_t)(uintptr_t)pthread_self();