feat(deps): add support for Nix package manager

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 SDK's, so builds run against
macOS 11.x SDKs via Nix. This should in theory allow the resulting
Emacs.app builds should be compatible with older macOS versions. Exactly
how well that holds up in practice remains to be seen.

Nix does support customizing the Apple SDK version, but I have not yet
figured out how to do so via a flake, and it's not a priority at the
moment, as the default v11 SDK is sufficient.

README.md

@@ -37,6 +37,18 @@ The build produced does have some limitations:
 ## Requirements
 
 - [Xcode](https://apps.apple.com/gb/app/xcode/id497799835?mt=12)
+
+# Nix
+
+The [Nix](https://nixos.org/) package manager is the preferred and most reliable
+way to install all dependencies required to build Emacs. Alternatively you can
+also use Homebrew.
+
+### Homebrew
+
+If you do not have Nix installed, then the alternative way to manage and install
+all dependencies is via Homebrew.
+
 - [Homebrew](https://brew.sh/)
 - Ruby 2.3.0 or later is needed to execute the build script itself. macOS comes
   with Ruby, check your version with `ruby --version`. If it's too old, you can
@@ -71,17 +83,38 @@ Nightly builds are built with GitHub Actions on GitHub-hosted runners, using
 
 ## Usage
 
+### Nix
+
+Ensure [Flakes](https://nixos.wiki/wiki/Flakes) are enabled, and enter the flake
+development environment with `nix develop`. Within this environment, you can
+execute the `./build-emacs-for-macos --help` to get started.
+
+Or you can run the build script via `nix develop`:
+
+```
+nix develop --command ./build-emacs-for-macos --help
+```
+
+### Homebrew
+
+Run `make boostrap` to ensure all Ruby and Homebrew dependencies are installed.
+
+### Build Script
+
 ```
 Usage: ./build-emacs-for-macos [options] <branch/tag/sha>
 
 Branch, tag, and SHA are from the emacs-mirror/emacs/emacs Github repo,
 available here: https://github.com/emacs-mirror/emacs
 
 Options:
+        --info                       Print environment info and detected library paths, then exit
+        --preview                    Print preview details about build and exit.
     -j, --parallel COUNT             Compile using COUNT parallel processes (detected: 16)
         --git-sha SHA                Override detected git SHA of specified branch allowing builds of old commits
+        --[no-]use-nix               Use Nix instead of Homebrew to find dependencies (default: enabled if EMACS_BUILD_USE_NIX is set to truthy value)
         --[no-]xwidgets              Enable/disable XWidgets if supported (default: enabled)
-        --[no-]tree-sitter           Enable/disable tree-sitter if supported (default: enabled)
+        --[no-]tree-sitter           Enable/disable tree-sitter if supported(default: enabled)
         --[no-]native-comp           Enable/disable native-comp (default: enabled if supported)
         --[no-]native-march          Enable/disable -march=native CFLAG(default: disabled)
         --[no-]native-full-aot       Enable/disable NATIVE_FULL_AOT / Ahead of Time compilation (default: disabled)