fix thread ID to be displayed on OS X (fix #874)

This patch fixes #874.

Thread number is now printed correctly (instead of -1) on OS X:

```
2014-11-19 17:22:15,492 365180 INFO  [server_util.cpp:361] starting jubaclassifier 0.6.4 RPC server at 192.168.21.1:9199
2014-11-19 17:22:30,349 365192 INFO  [server_base.cpp:129] starting save to /private/tmp/192.168.21.1_9199_classifier_foo.jubatus
```

This value matches with the thread ID (`tid`) shown in lldb, so it is useful for debugging:

```
(lldb) thread list
Process 2856 stopped
* thread #1: tid = 0x5927c, 0x00007fff89a0aa3a libsystem_kernel.dylib`__semwait_signal + 10, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
  thread #2: tid = 0x59286, 0x00007fff89a0acc2 libsystem_kernel.dylib`__sigwait + 10
  thread #3: tid = 0x59287, 0x00007fff89a0b64a libsystem_kernel.dylib`kevent + 10
  thread #4: tid = 0x59288, 0x00007fff89a0a716 libsystem_kernel.dylib`__psynch_cvwait + 10
```

How about stop using ```#define```?

```
namespace {
#ifdef __APPLE__
const int gettid = SYS_thread_selfid
#else
...
}
```