This serves as an alternative to Homebrew. It should be much more stable
and cause less headaches over time for automated builds.
There should be no change to the end user experience of using the build
script, as it should still work with and use Homebrew by default.
Additionally, Nix provides older Apple SDKs, allowing us to run against
macOS 11.x SDKs. This allows the resulting Emacs.app builds to be
compatible with macOS 11.x and later versions.
In testing, this seems to be the case on macOS 11.x (x86_64) and macOS
12.x (arm64).
This is a near complete rewrite of the shared library embedder/bundler
code. It now correctly resolves shared libraries that link to other
shared library files in the same package via use of `@path`, which the
libwebp package does extensively between `libwebpdemux.2.dylib`,
`libwebp.7.dylib` and `libsharpyuv.0.dylib`.
These relative shared library links were not understood at all by the
old library bundler code.
The new replacement here fully understands and resolves all
`@<something>` placeholders in links to shared libraries. It operates in
a two-step process where it first walks down the three of all shared
libraries that Emacs links to, and all that they link to, etc. building
a copy and relink plan.
In a second step it then executes the copy plan, followed by the
relinking plan.
The result is a fully self-contained Emacs.app binary, which has no
dependencies on Homebrew or anything installed via Homebrew. Builds have
been tested on a fresh install of macOS Sonoma VM, with nothing but
Xcode Command Line Tools installed. Everything from native compilation
to webp, svg, png rendering, and more worked as expected.
