Build on ARM Mac

Latest MacOS github runner is ARM-based! See the announcement here:
https://github.blog/2023-10-02-introducing-the-new-apple-silicon-powered-m1-macos-larger-runner-for-github-actions/.

MacOS 14 and higher will only be provided in ARM versions from now on. The
software listed for 13 and higher
(https://github.com/actions/runner-images/blob/macOS-12/20240412.2/images/macos/macos-14-arm64-Readme.md)
does not include Tcl or SWIG, so we have to install these manually.

.github/workflows/build.yml

@@ -27,6 +27,8 @@ jobs:
           ubuntu-latest,
           # latest available X86_64 target
           macos-12,
+          # latest is ARM
+          macos-latest,
         ]
     steps:
       - name: Checkout
@@ -49,6 +51,14 @@ jobs:
         if: matrix.os == 'ubuntu-latest'
         run: sudo apt-get update && sudo apt-get install tcl-dev
 
+      - name: Setup tcl (macos-latest)
+        if: matrix.os == 'macos-latest'
+        run: brew install tcl-tk
+
+      - name: Setup SWIG (macos-latest)
+        if: matrix.os == 'macos-latest'
+        run: brew install swig
+
       - name: build
         env:
           # instructs SCons to print debug statements to stdout