add pthread_once(3) implementation for Windows

include/compat/pthread.h

@@ -0,0 +1,43 @@
+/*
+ * Public domain
+ * pthread.h compatibility shim
+ */
+
+#ifdef _WIN32
+
+#include <synchapi.h>
+
+/*
+ * Static once initialization values.
+ */
+#define PTHREAD_ONCE_INIT   { INIT_ONCE_STATIC_INIT }
+
+/*
+ * Once definitions.
+ */
+struct pthread_once {
+	INIT_ONCE once;
+};
+
+typedef struct pthread_once pthread_once_t;
+
+static BOOL CALLBACK _pthread_once_win32_cb(PINIT_ONCE once, PVOID param, PVOID *context)
+{
+	void (*cb) (void) = param;
+	cb();
+    return TRUE;
+}
+
+static int pthread_once(pthread_once_t *once, void (*cb) (void))
+{
+	BOOL rc = InitOnceExecuteOnce(&once->once, _pthread_once_win32_cb, cb, NULL);
+	if (rc == 0) {
+		return -1;
+	} else {
+		return 0;
+	}
+}
+
+#else
+#include_next <pthread.h>
+#endif