time.h on mac

[xor256]

cc -g -c -o main.o main.c -ltime
clang: warning: -ltime: 'linker' input unused
main.c:51:5: warning: implicit declaration of function 'clock_gettime' is
invalid in C99 [-Wimplicit-function-declaration]
clock_gettime(CLOCK_REALTIME, &start);
^
main.c:51:19: error: use of undeclared identifier 'CLOCK_REALTIME'
clock_gettime(CLOCK_REALTIME, &start);
^
main.c:54:19: error: use of undeclared identifier 'CLOCK_REALTIME'
clock_gettime(CLOCK_REALTIME, &end);
^
1 warning and 2 errors generated.