pthread_once: Add static assertions for ONCE_INITs.

Both of these need to be here, they cannot be in the C sources,
as the static assertions throw "non-constant expression" when
placed there.

src/system/libroot/posix/pthread/pthread_once.cpp

@@ -1,11 +1,14 @@
 /*
  * Copyright 2008-2011, Ingo Weinhold, [email protected].
+ * Copyright 2023, Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  */
 
 #include <pthread.h>
+#include <threads.h>
 
 #include <OS.h>
+#include <Debug.h>
 
 
 enum {
@@ -15,6 +18,11 @@ enum {
 	STATE_INITIALIZED	= -4
 };
 
+#if __cplusplus >= 201103L
+STATIC_ASSERT(((pthread_once_t)ONCE_FLAG_INIT).state == ((pthread_once_t)PTHREAD_ONCE_INIT).state);
+STATIC_ASSERT(((pthread_once_t)PTHREAD_ONCE_INIT).state == STATE_UNINITIALIZED);
+#endif
+
 
 /*!	Called when the thread performing the initialization function was canceled.