c11 error: undefining "__STDC_NO_THREADS__"

#if !defined(THREAD_FALLBACK) && defined(__STDC_NO_THREADS__)

#define THREAD_FALLBACK

//#ifdef __STDC_NO_THREADS__

//#undef __STDC_NO_THREADS__

#include <pthread.h>

#include <errno.h>

#define thrd_t pthread_t

#define thrd_create( thr, func, arg ) pthread_create( thr, 0x0, func, arg )

#define thrd_equal( lhs, rhs ) pthread_equal( lhs, rhs )

#define thrd_current() pthread_self()

#define thrd_sleep( time_point, remaining ) nanosleep( time_point, remaining )

#define thrd_yield() sched_yield()

#define thrd_exit( res ) pthread_exit( res )

#define thrd_detach( thr ) pthread_detach( thr )

#define mtx_t mutex_wrapper_t

#define mtx_lock( mutex ) pthread_mutex_lock( mutex.mutex )

#define mtx_trylock( mutex ) mtx_trylock( mutex.mutex )

#define mtx_unlock( mutex ) pthread_mutex_unlock( mutex.mutex )

#define mtx_destroy( mutex ) ((mutex.type&&pthread_mutexattr_destroy( mutex.attribute )),pthread_mutex_destroy( mutex.mutex ))

enum {

  thrd_success = 0,

  thrd_nomem = EAGAIN,

  thrd_timedout = ETIMEDOUT,

  thrd_busy = EBUSY,

  thrd_error = ~0

};

typedef struct {

  pthread_mutex_t mutex;

  pthread_mutexattr_t attribute;

  int type;

} mutex_wrapper_t;

enum {

  mtx_plain     = (1<<1),

  mtx_recursive = (1<<2),

  mtx_timed     = (1<<3)

};

inline int mtx_init( mtx_t* mutex, int type ){

  mutex->type = type;

  switch(type){

    case 0: break;

    case mtx_plain:

    case mtx_recursive:

    case mtx_timed:

      pthread_mutexattr_init( &mutex->attribute );

    break;

    default: return -1;

  }

  pthread_mutex_init( &mutex->mutex, &mutex->attribute );

}

#endif