How to evaluate a program's runtime?

I've developed a simple program and want to evaluate its runtime performance on a real machine, e.g. my MacBook. The source code goes:

#include <stdio.h>
#include <vector>
#include <ctime>

int main () {
    auto beg = std::clock () ;
    for (int i = 0; i < 1e8; ++ i) {

    }
    auto end = std::clock () ;
    printf ("CPU time used: %lf ms\n", 1000.0*(end-beg)/CLOCKS_PER_SEC) ;
}

It's compiled with gcc and the optimization flag is set to the default. With the help of bash script, I ran it for 1000 times and recorded the runtime by my MacBook, as following:

[130.000000, 136.000000): 0
[136.000000, 142.000000): 1
[142.000000, 148.000000): 234
[148.000000, 154.000000): 116
[154.000000, 160.000000): 138
[160.000000, 166.000000): 318
[166.000000, 172.000000): 139
[172.000000, 178.000000): 40
[178.000000, 184.000000): 11
[184.000000, 190.000000): 3

"[a, b): n" means that the actual runtime of the same program is between a ms and b ms for n times.

It's clear that the real runtime varies greatly and it seems not a normal distribution. Could someone kindly tell me what causes this and how I can evaluate the runtime correctly?

Thanks for responding to this question.