vp9_thread: prefer pthread.h if available

this avoids conflicts with recent versions of mingw-w64 (tested g++
4.8.2) and the unit tests

Change-Id: Ic41ea31eebe0e3e712ed5e657f37d8cad6712088

vp9/common/vp9_thread.h

@@ -28,7 +28,7 @@ extern "C" {
 
 #if CONFIG_MULTITHREAD
 
-#if defined(_WIN32)
+#if defined(_WIN32) && !HAVE_PTHREAD_H
 #include <errno.h>  // NOLINT
 #include <process.h>  // NOLINT
 #include <windows.h>  // NOLINT