cirrus.yml: add latest Xcode build target

CirrusCI provides a mojave-xcode alias for the latest Xcode available.
Let's use it to make sure we track the latest releases.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.cirrus.yml

@@ -25,3 +25,14 @@ macos_task:
     - ./configure --python=/usr/local/bin/python3 || { cat config.log; exit 1; }
     - gmake -j$(sysctl -n hw.ncpu)
     - gmake check -j$(sysctl -n hw.ncpu)
+
+macos_xcode_task:
+  osx_instance:
+    # this is an alias for the latest Xcode
+    image: mojave-xcode
+  install_script:
+    - brew install pkg-config gnu-sed glib pixman make sdl2
+  script:
+    - ./configure --cc=clang || { cat config.log; exit 1; }
+    - gmake -j$(sysctl -n hw.ncpu)
+    - gmake check -j$(sysctl -n hw.ncpu)