patch 8.1.2352: CI doesn't cover FreeBSD

Problem:    CI doesn't cover FreeBSD.
Solution:   Configure Cirrus-CI. (Christian Brabandt, closes #5273)
diff --git a/.cirrus.yml b/.cirrus.yml
new file mode 100644
index 0000000..0e4a3d6
--- /dev/null
+++ b/.cirrus.yml
@@ -0,0 +1,9 @@
+freebsd_12_task:
+  freebsd_instance:
+    image: freebsd-12-1-release-amd64
+  install_script:
+    pkg install -y gettext gmake
+  build_script:
+    - gmake
+  test_script:
+    - gmake test