Fix leaking of FDs created by pipe

[Vogtinator]

Use pipe2 to pass O_CLOEXEC.

[hanna-kn]

If I'm not mistaken, pipe2 is not very portable. There is a different implementation of the Thread class in libqalculate for win32, but the code must work for MacOS and other non-Linux systems.

[Vogtinator]

Any other implementation, like using fcntl, would be inherently racy.

So I guess using autotools feature detection is the best way here. I'm not very familiar with autotools though.

[hanna-kn]

I just need to add AC_CHECK_FUNCS([pipe2]) to configure.ac and use #ifdef HAVE_PIPE2.

I will merge your changes and then add the checks.

[Vogtinator]

Thanks!