It was introduced in Xcode 9 and made the default in Xcode 10. We were forcing
the use of the legacy version, which doesn't support some new features that we
wish to enable, such as building the SDK for distribution.

Xcode 10 introduced a new build system. Alas, it breaks a number of important
flows, such as creating an archive for the framework (ie SDK) target.

In order to "fix" this, switch back to the former (Xcode 9) build system for the
time being.

Looks like it's a good idea to check it in: https://stackoverflow.com/a/49564624