Increase thread stack for OS X (#2035)

On OS X threads other than the main thread are created with a reduced stack
size of 512KB by default, this is dangerously low for deep searches, so
adjust it to TH_STACK_SIZE. The implementation calls pthread_create() with
proper stack size parameter.

Verified for no regression at STC enabling the patch on all platforms where
pthread is supported.

LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 50873 W: 9768 L: 9700 D: 31405

No functional change.

src/thread.h

@@ -32,7 +32,7 @@
 #include "pawns.h"
 #include "position.h"
 #include "search.h"
-#include "thread_win32.h"
+#include "thread_win32_osx.h"
 
 
 /// Thread class keeps together all the thread-related stuff. We use
@@ -46,7 +46,7 @@ class Thread {
   ConditionVariable cv;
   size_t idx;
   bool exit = false, searching = true; // Set before starting std::thread
-  std::thread stdThread;
+  NativeThread stdThread;
 
 public:
   explicit Thread(size_t);