Always use pthread

After a858defd33, the pthread implementation of `NativeThread` is always used (except MSVC), as:

1. `-DUSE_PTHREADS` is defined when `comp` is not `mingw`
2. the pthread implementation is used anyways because MinGW defines `__MINGW32__` or `__MINGW64__` macro

closes https://github.com/official-stockfish/Stockfish/pull/6985

No functional change

src/Makefile

@@ -90,7 +90,7 @@ HEADERS = attacks.h benchmark.h bitboard.h evaluate.h misc.h movegen.h movepick.
 		nnue/layers/affine_transform.h nnue/layers/affine_transform_sparse_input.h \
 		nnue/layers/clipped_relu.h nnue/layers/sqr_clipped_relu.h nnue/nnue_accumulator.h \
 		nnue/nnue_architecture.h nnue/nnue_common.h nnue/nnue_feature_transformer.h nnue/simd.h \
-		nnue/nnz_helper.h position.h search.h syzygy/tbprobe.h thread.h thread_win32_osx.h timeman.h \
+		nnue/nnz_helper.h position.h search.h syzygy/tbprobe.h thread.h thread_native.h timeman.h \
 		tt.h tune.h types.h uci.h ucioption.h perft.h nnue/network.h engine.h score.h numa.h memory.h shm.h shm_linux.h
 
 OBJS = $(notdir $(SRCS:.cpp=.o))
@@ -685,9 +685,6 @@ else ifeq ($(COMP),mingw)
 	CXXFLAGS += -fno-ipa-cp-clone
 endif
 
-### On mingw use Windows threads, otherwise POSIX
-ifneq ($(comp),mingw)
-	CXXFLAGS += -DUSE_PTHREADS
 # On Android Bionic's C library comes with its own pthread implementation bundled in
 ifneq ($(OS),Android)
 	# Haiku has pthreads in its libroot, so only link it in on other platforms
@@ -712,7 +709,6 @@ ifneq ($(comp),mingw)
 		endif
 	endif
 endif
-endif
 
 ### 3.2.1 Debugging
 ifeq ($(debug),no)

src/thread.h

@@ -31,7 +31,7 @@
 #include "numa.h"
 #include "position.h"
 #include "search.h"
-#include "thread_win32_osx.h"
+#include "thread_native.h"
 
 namespace Stockfish {
 

src/thread_native.h

@@ -16,10 +16,29 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#ifndef THREAD_WIN32_OSX_H_INCLUDED
-#define THREAD_WIN32_OSX_H_INCLUDED
+#ifndef THREAD_NATIVE_H_INCLUDED
+#define THREAD_NATIVE_H_INCLUDED
 
+#ifdef _MSC_VER
     #include <thread>
+#else
+    #include <pthread.h>
+    #include <functional>
+    #include <utility>
+
+    #include "misc.h"
+#endif
+
+namespace Stockfish {
+
+#ifdef _MSC_VER
+
+// MSVC-compatible toolchains use std::thread because they do not provide
+// pthreads by default. On all other platforms, pthreads is required and used.
+
+using NativeThread = std::thread;
+
+#else
 
 // On OSX threads other than the main thread are created with a reduced stack
 // size of 512KB by default, this is too low for deep searches, which require
@@ -27,13 +46,6 @@
 // The implementation calls pthread_create() with the stack size parameter
 // equal to the Linux 8MB default, on platforms that support it.
 
-#if defined(__APPLE__) || defined(__MINGW32__) || defined(__MINGW64__) || defined(USE_PTHREADS)
-
-    #include <pthread.h>
-    #include <functional>
-
-namespace Stockfish {
-
 class NativeThread {
     pthread_t thread;
 
@@ -63,16 +75,8 @@ class NativeThread {
     void join() { pthread_join(thread, nullptr); }
 };
 
-}  // namespace Stockfish
-
-#else  // Default case: use STL classes
-
-namespace Stockfish {
-
-using NativeThread = std::thread;
+#endif  // _MSC_VER
 
 }  // namespace Stockfish
 
-#endif
-
-#endif  // #ifndef THREAD_WIN32_OSX_H_INCLUDED
+#endif  // #ifndef THREAD_NATIVE_H_INCLUDED