cc -O2 -pipe  -pthread  -Wall -Wextra -Werror=implicit-function-declaration time/clock_nanosleep.c -o time/clock_nanosleep -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm -lpthread 
time/clock_nanosleep.c:12:7: error: call to undeclared function 'clock_nanosleep'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
        if ( clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &now, NULL) < 0 )
             ^
1 error generated.