add Github actions workflow for FreeBSD build

.github/workflows/main.yml

@@ -0,0 +1,35 @@
+name: build
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  bsd:
+    runs-on: ubuntu-latest
+    name: Main build job
+    steps:
+    - uses: actions/checkout@v4
+    - name: FreeBSD job
+      id: test
+      uses: vmactions/freebsd-vm@v1
+      with:
+        usesh: true
+        sync: sshfs
+        prepare: |
+          pkg install -y git cmake gmake
+        run: |
+          mkdir build
+          cd build
+          cmake ..
+          gmake all -j6
+    - name: Collect outputs
+      run: |
+        mkdir _output
+        cp build/bin/* _output
+    - name: Upload
+      uses: actions/upload-artifact@v4
+      with:
+        name: output_bsd
+        path: _output