ci: add FreeBSD build CI and glog 0.7.x compatibility

Add GitHub Actions workflow for FreeBSD using vmactions/freebsd-vm@v1.
This runs cmake + make on FreeBSD 14.2 with system-installed deps.

Also add conditional GLOG_USE_GLOG_EXPORT detection for glog >= 0.7
compatibility. This uses check_include_file_cxx to detect glog/export.h
(added in glog 0.7) before defining GLOG_USE_GLOG_EXPORT. Safe with
older glog versions where the header does not exist. This benefits all
platforms, not just FreeBSD.

Author: tuaris

.github/workflows/ci-freebsd.yml

@@ -0,0 +1,33 @@
+name: Build on FreeBSD
+
+on:
+  push:
+    branches: [ master ]
+    paths-ignore:
+      - '**.md'
+  pull_request:
+    branches: [ master ]
+    paths-ignore:
+      - '**.md'
+
+jobs:
+  compile-with-cmake:
+    runs-on: ubuntu-latest
+    name: FreeBSD build
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Build on FreeBSD
+      uses: vmactions/freebsd-vm@v1
+      with:
+        release: "14.2"
+        usesh: true
+        prepare: |
+          pkg install -y cmake protobuf gflags leveldb openssl git
+
+        run: |
+          set -ex
+          mkdir build && cd build
+          cmake -DCMAKE_PREFIX_PATH=/usr/local ..
+          make -j$(sysctl -n hw.ncpu)