Ch. 3 Build system should mention how to link to zig libraries and C libraries and libc

[pjz]

Setting aside the lack of a standard package/module/library manager, it would be nice to have at least minimal docs on exe.addPackage(), exe.linkLibC(), exe.linkSystemLibrary().

[pjz]

Based on:

• https://github.com/ziglang/zig/wiki/Zig-Build-System
• Links from https://www.reddit.com/r/Zig/comments/jb8qd3/resources_for_understanding_zig_build_system/