Add FreeBSD build on Cirrus CI

.ci/unix-build.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+mkdir build && cd build
+cmake -DCMAKE_BUILD_TYPE=Release ..
+cmake --build .

.ci/unix-test.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+cd build
+ctest -E Windows
+if [ -f "test/std_filesystem_test" ]; then
+  test/std_filesystem_test || true
+fi

.cirrus.yml

@@ -0,0 +1,13 @@
+freebsd_instance:
+  image_family: freebsd-12-1
+
+task:
+  install_script: |
+    pkg install -y cmake
+    pw groupadd testgrp
+    pw useradd testuser -g testgrp -w none -m
+    chown -R testuser:testgrp .
+  build_script: |
+    sudo -u testuser .ci/unix-build.sh
+  test_script: |
+    sudo -u testuser .ci/unix-test.sh