Why isn't "Swift Compiler - Custom Flags" setting showing up in build settings?

Question

I am working on an Xcode project that uses a Calendar framework, here: https://github.com/mmick66/CalendarView.

In the readme is says in order to use the Event system, you need to set custom swift compiler flags in the build settings. For some reason it isn't shown as an option for me.

Here is a screenshot of what I see in my projects build settings.

On the repository is a sample project. When I open that in Xcode, "Swift Compiler - Custom Flags" is one of the sections unlike my project.

Why would this be happening?

Here is a screenshot from the open source example project:

Answer 1

As per Richard Robinson's comment. I was able to find the required settings by restarting Xcode, and in the Build Settings tab, click all (combined is default) then searching for "custom flags". From there I was able to add the required flags for the calendar framework.

Edit 1: I jus thought I should add this clause to this answer. This doesn't technically answer the "why" is this happening, rather just how to fix it.