> Why is it important that the build host is portable?

The person you’re replying to did not say the build host has to be portable. They said that the software configuration script needed to be portable:

> And [for `configure`] to be portable to such systems you needed to check everything.

Edit:

> Why not choose a single platform for your CI and build for all of your targets on those using a compatibility library to cover up differences.

You can. Nothing precludes you from doing this. But there are situations where you control neither the build environment nor the install environment.