ci: add a crossbuild workflow

.github/workflows/crossbuild.yml

@@ -0,0 +1,41 @@
+name: Cross-build openrsync
+on:
+  push:
+    branches: ['*']
+  pull_request:
+    types: [opened, reopened]
+permissions:
+  contents: read
+jobs:
+  build:
+    name: ${{ matrix.os }} (${{ matrix.compiler }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-20.04, ubuntu-22.04, macos-latest]
+        include:
+          - os: ubuntu-20.04
+            compiler: clang12
+            pkgs: clang-12 lld-12
+          - os: ubuntu-22.04
+            compiler: clang14
+            pkgs: clang-14 lld-14
+          - os: macos-latest
+            compiler: clang13
+            pkgs: llvm@13
+    steps:
+      - uses: actions/checkout@v3
+      - name: install packages (Ubuntu)
+        if: runner.os == 'Linux'
+        run: |
+          sudo apt-get update --quiet || true
+          sudo apt-get -yq --no-install-suggests --no-install-recommends install ${{ matrix.pkgs }}
+      - name: install packages (macOS)
+        if: runner.os == 'macOS'
+        run: |
+          brew update --quiet || true
+          brew install ${{ matrix.pkgs }} || true
+      - name: configure
+        run: env CC=${{ matrix.compiler }} ./configure
+      - name: build
+        run: make