Build best practices for GLib and GTK

Instead of manually defining preprocessor flags that we wish to use, let's trust upstream to pick good defaults for us. This follows Rule 2 from my blog post:

https://blogs.gnome.org/mcatanzaro/2022/07/15/best-practices-for-build-options/

We should follow up in GTK itself to make sure GTK's defaults are good.

This partially reverts !1590 (merged)

Also:

GLib upstream has chosen to leave this flag off by default in production builds. We should match upstream defaults.

Also, manually make GTK 3 builds match GTK 4 builds:

GTK 4 production builds will not enable debug or asserts.

Certainly, it does not make sense to enable debug in our builds when upstream has chosen not to.