github actions: add build description

Add basic builds on Ubuntu and macOS hosts for Github's CI
automation. Each builds with either GNU Autotools or the
plain Makefile, and then builds the documentation.

Based on similar code from libopusenc, borrowing steps
from the ci scripts directory and .travis-ci.yml.

.github/workflows/action.yml

@@ -0,0 +1,75 @@
+name: GitHub CI
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: '0 0 1 * *'
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        name:
+          [
+            ubuntu-autotools,
+            ubuntu-makefile,
+            macos-autotools,
+            macos-makefile,
+          ]
+        include:
+
+          - name: ubuntu-autotools
+            os: ubuntu-latest
+            build-system: autotools
+
+          - name: ubuntu-makefile
+            os: ubuntu-latest
+            build-system: makefile
+
+          - name: macos-autotools
+            os: macos-latest
+            build-system: autotools
+
+          - name: macos-makefile
+            os: macos-latest
+            build-system: makefile
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Install Linux dependencies
+        if: startsWith(matrix.os,'ubuntu')
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libopus-dev libogg-dev libssl-dev
+
+      - name: Install MacOS dependencies
+        if: startsWith(matrix.os,'macos')
+        run: |
+          brew bundle
+
+      - name: Build with Autotools
+        if: startsWith(matrix.build-system,'autotools')
+        run: |
+          ./autogen.sh
+          ./configure
+          make
+          make check
+
+      - name: distcheck with Autotools
+        if: startsWith(matrix.build-system,'autotools')
+        run: |
+          make distcheck
+
+      - name: Build with Makefile
+        if: startsWith(matrix.build-system,'makefile')
+        run: |
+          make -C unix
+          make -C unix check
+
+      - name: Build Documentation
+        run: |
+          make -C doc