CI, macOS: install autoconf.

Reference issue: https://github.com/libsdl-org/SDL/issues/9606 .

(cherry picked from commit 26e3d65ea8)

.github/workflows/main.yml

@@ -64,6 +64,7 @@ jobs:
       if: runner.os == 'macOS'
       run: |
         brew install \
+          autoconf \
           ninja \
           pkg-config