ci: Test *BSD build

Tests on OpenBSD, FreeBSD and NetBSD. Both x86-64 and arm64 are used.

.github/workflows/build_bsd.yml

@@ -0,0 +1,54 @@
+name: "Build on *BSD"
+
+on:
+  push: {}
+  pull_request: {}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os:
+          - name: openbsd
+            architecture: x86-64
+            version: '7.5'
+
+              #- name: openbsd
+              #  architecture: arm64
+              #  version: '7.6'
+
+          - name: freebsd
+            architecture: x86-64
+            version: '14.0'
+
+              #- name: freebsd
+              #  architecture: arm64
+              #  version: '14.1'
+
+          - name: netbsd
+            architecture: x86-64
+            version: '9.4'
+
+              #- name: netbsd
+              #  architecture: arm64
+              #  version: '10.0'
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build on ${{ matrix.os.name }} / ${{ matrix.os.architecture }}
+        uses: cross-platform-actions/action@v0.25.0
+        with:
+          environment_variables: MY_ENV1 MY_ENV2
+          operating_system: ${{ matrix.os.name }}
+          architecture: ${{ matrix.os.architecture }}
+          version: ${{ matrix.os.version }}
+          shell: bash
+          memory: 5G
+          cpu_count: 4
+          run: |
+            pkg install autoconf
+            autoreconf -vfi
+            ./configure
+            make