make target specific dependencies lazy

[Jan200101]

spiritual follow-up to #1447

turns zig_objc and zig_js into lazy dependencies so they aren't required when building for different targets using the system package mode

I don't have a mac to test this so I'm at the mercy of someone else doing that for me

[der-teufel-programming]

Great idea

[Jan200101]

It appears as if making a dependency lazy stops zig fetch from automatically downloading it.
This hasn't been an issue with GLFW since its not used as default anywhere but making zig_objc and zig_js into lazy dependencies will possibly break Nix builds on macOS.