Add info about the -fno-sys=gtk4-layer-shell build flag to the "build from source" documentation

[arizon-dread]

Issue

I am running Linux Mint 22.1 and I don't have the gtk4-layer-shell binary in the repo. I found out through a help-thread I started on discord that this flag (-fno-sys=gtk4-layer-shell) exists.

Possible solution

I would like this section: build#linux to contain info that if the linux dist repo doesn't ship the gtk4-layer-shell binary, you can build with the flag to build the dependency when building ghostty.

Suggested text for the documentation

💡 Tip
If your Linux distribution's package repository does not provide gtk4-layer-shell you can add this flag to the zig build command: -fno-sys=gtk4-layer-shell to build the dependency when Ghostty builds.

[pluiedev]

This is something I'm planning to include in the release notes for 1.2.0, but I figure we can add it to PACKAGING.md too

[arizon-dread]

That would be great for building binaries for public repos, but would not help if building from source for your own machine (when following the build from source documentation), that's why I wanted it in the general documentation where the dependencies are listed.

[pluiedev]

Looking more at it, the section needs to be rewritten entirely anyways because in 1.2.0 we want to encourage more people to use the release tarballs instead of cloning the Git repo. (#7316)

[pluiedev]

Opened ghostty-org/website#358