Add hg_thread_mutex initializer
#include "mercury_util_config.h"
#ifdef _WIN32
#include <windows.h>
+ #define HG_THREAD_MUTEX_INITIALIZER NULL
typedef CRITICAL_SECTION hg_thread_mutex_t;
#else
#include <pthread.h>
#include <errno.h>
+ #define HG_THREAD_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
typedef pthread_mutex_t hg_thread_mutex_t;
#endif
