cc -O2 -pipe  -pthread  -Wall -Wextra -Werror=implicit-function-declaration pthread/pthread_cond_clockwait.c -o pthread/pthread_cond_clockwait -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm -lpthread 
pthread/pthread_cond_clockwait.c:20:16: error: call to undeclared function 'pthread_cond_clockwait'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
   20 |         if ( (errno = pthread_cond_clockwait(&cnd, &mtx, CLOCK_MONOTONIC,
      |                       ^
1 error generated.