Use pthreads on MinGW.

Android uses MinGW for some host tools on Windows. That toolchain
doesn't support the #pragma tricks we use for thread-local destructors,
but does appear to support pthreads.

This also lets us remove the INIT_ONCE workaround, although that's
removable anyway since Android's MinGW is now new enough.

Change-Id: I8d1573923fdaac880a50d84acbebbf87461c50d2
Reviewed-on: https://boringssl-review.googlesource.com/11125
Reviewed-by: David Benjamin <davidben@google.com>
Reviewed-by: Kenny Root <kroot@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/crypto/thread_win.c b/crypto/thread_win.c
index c7a90f7..836cf0f 100644
--- a/crypto/thread_win.c
+++ b/crypto/thread_win.c
@@ -14,7 +14,7 @@
 
 #include "internal.h"
 
-#if defined(OPENSSL_WINDOWS) && !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_WINDOWS_THREADS)
 
 OPENSSL_MSVC_PRAGMA(warning(push, 3))
 #include <windows.h>
@@ -234,4 +234,4 @@
   return 1;
 }
 
-#endif  /* OPENSSL_WINDOWS && !OPENSSL_NO_THREADS */
+#endif  /* OPENSSL_WINDOWS_THREADS */