Avoid using both Win32Waiter and PthreadWaiter on MinGW,

and use StdcppWaiter instead.

There are various flavors of MinGW, some of which support pthread,
and some of which support Win32. Instead of figuring out which
platform is being used, just use the generic implementation.

PiperOrigin-RevId: 580565507
Change-Id: Ia85fd7496f1e6ebdeadb95202f0039e844826118

absl/synchronization/internal/pthread_waiter.h

@@ -16,7 +16,7 @@
 #ifndef ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
 #define ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
 
-#ifndef _WIN32
+#if !defined(_WIN32) && !defined(__MINGW32__)
 #include <pthread.h>
 
 #include "absl/base/config.h"
@@ -55,6 +55,6 @@ class PthreadWaiter : public WaiterCrtp<PthreadWaiter> {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ndef _WIN32
+#endif  // !defined(_WIN32) && !defined(__MINGW32__)
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_

absl/synchronization/internal/win32_waiter.h

@@ -20,7 +20,8 @@
 #include <sdkddkver.h>
 #endif
 
-#if defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
+#if defined(_WIN32) && !defined(__MINGW32__) && \
+    _WIN32_WINNT >= _WIN32_WINNT_VISTA
 
 #include "absl/base/config.h"
 #include "absl/synchronization/internal/kernel_timeout.h"
@@ -65,6 +66,7 @@ class Win32Waiter : public WaiterCrtp<Win32Waiter> {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
+#endif  // defined(_WIN32) && !defined(__MINGW32__) &&
+        // _WIN32_WINNT >= _WIN32_WINNT_VISTA
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_WIN32_WAITER_H_