feat(deps): add support for Nix package manager (#116)

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).

README.md

@@ -36,18 +36,40 @@ The build produced does have some limitations:
 
 ## Requirements
 
+Required with both Nix and Homebrew approaches:
+
 - [Xcode](https://apps.apple.com/gb/app/xcode/id497799835?mt=12)
-- [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
-  install a newer version with:
-  ```
-  brew install ruby
-  ```
-- All dependencies can all easily be installed by running:
-  ```
-  make bootstrap
-  ```
+
+### Nix
+
+The [Nix](https://nixos.org/) package manager is the preferred and most reliable
+way to install all dependencies required to build Emacs, by way of a Nix flake
+included in the project root.
+
+To install all required dependencies within the nix shell, run:
+
+```
+nix develop --command make bootstrap
+```
+
+### Homebrew
+
+If you do not have Nix installed, then the alternative way to manage and install
+build-time dependencies is via [Homebrew](https://brew.sh/).
+
+Ruby 3.3.x or later is also needed to execute the build script. Earlier versions
+may work, but are untested. Simplest way to install a recent Ruby version is via
+Homebrew:
+
+```
+brew install ruby
+```
+
+And finally, to install all built-time dependencies, run:
+
+```
+make bootstrap
+```
 
 ## Status
 
@@ -71,6 +93,24 @@ 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>
 
@@ -78,8 +118,11 @@ 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 IN_NIX_SHELL is set)
         --[no-]xwidgets              Enable/disable XWidgets 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)