CI: Build test on FreeBSD via Cirrus-CI

Cirrus-CI is a hosted CI service that supports Linux, FreeBSD, Windows,
and macOS.  Add a Cirrus-CI config to start build testing on FreeBSD.

.cirrus.yml

@@ -0,0 +1,18 @@
+freebsd_instance:
+  image_family: freebsd-14-0
+
+task:
+  name: "Build"
+  install_script: >
+    pkg install -y cmake ninja pkgconf
+    graphics/png graphics/webp graphics/tiff graphics/libheif graphics/libavif
+    graphics/libimagequant print/freetype2 x11/libXpm x11-fonts/fontconfig
+    print/libraqm
+  script: |
+      mkdir build
+      cd build
+      cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Debug \
+      -DENABLE_PNG=1 -DENABLE_FREETYPE=1 -DENABLE_JPEG=1 -DENABLE_WEBP=1 \
+      -DENABLE_TIFF=1 -DENABLE_XPM=1 -DENABLE_GD_FORMATS=1 -DENABLE_HEIF=1 \
+      -DENABLE_RAQM=1 -DENABLE_AVIF=1 -DBUILD_TEST=1
+      ninja