Why is CLK_TCK undeclared although I included time.h?

I used the web editor ideone.com. I wrote the code shown below in C++. Using CLK_TCK, I attempted to measure the operation time of the function I defined (called "f1" in my code). As shown in below, in the "output" space of the web editor (maybe correspond to terminal in some editors), it said that CLK_TCK is undefined and it advised me to use CLOCK_TAI instead.

Would you please answer the following two questions regarding this?

Question1: Why is CLK_TCK undefined in my case?

Question2: What is the difference between CLK_TCK and CLOCK_TAI?

The code I wrote is shown below.

#include <stdio.h>
#include <time.h>
#include <math.h>
clock_t start, stop;
double duration;
#define MAXN 10
double f1 (int n, double a[], double x) {
    int i;
    double p=a[0];
    for (i=1; i<=n; i++)
     p+=(a[i]*pow(x,i));
     return p;
    }

int main() {
    int i;
    double a[MAXN];
    for (i=0;i<MAXN;i++) a[i]=(double)i;

    start=clock();
    f1(MAXN-1,a,1.1);
    stop=clock();
    duration=((double)(stop-start))/CLK_TCK;
    printf("ticks1 = %f\n", (double)(stop-start));
    printf("durationa1=%6.2e\n",duration);
    
    return 0;
}

The sentences shown in the output space in the web editor.

Compilation error #stdin compilation error #stdout 0s 5532KB
prog.cpp: In function ‘int main()’:
prog.cpp:29:34: error: ‘CLK_TCK’ was not declared in this scope
  duration=((double)(stop-start))/CLK_TCK;
                                  ^~~~~~~
prog.cpp:29:34: note: suggested alternative: ‘CLOCK_TAI’
  duration=((double)(stop-start))/CLK_TCK;
                                  ^~~~~~~
                                  CLOCK_TAI