title: Create child process with custom environment using boost
body: <p>The documentation <a href="https://www.boost.org/doc/libs/1_64_0/doc/html/process.html" rel="nofollow noreferrer">boost</a> doesn't provide any example for creating a child process with a custom environment using <code>process::child(...)</code>.<br>
  An example is given with <code>process::system(...)</code> but the function <code>system</code> has less possible operations (such as pipes or waitpid) so I would like to have a full example using <code>process::child</code> if possible.</p>