gitlab-ci: Add 32bit MinGW jobs using MSYS2

This builds glib using meson/ninja/ccache with mingw-w64 on a Windows
machine.

The CI scripts expect a gitlab runner to exist with the "win32" tag
which uses the default "cmd" shell by default.

Before running the tests pacman is invoked to update the system
(potentially including bash etc, thus the extra step)
Then a login shell is started with CHERE_INVOKING to not change the
cwd and finally the test script is executed.

https://bugzilla.gnome.org/show_bug.cgi?id=793729

.gitlab-ci.yml

@@ -3,6 +3,10 @@ image: ebassi/gitlab-glib:latest
 stages:
   - build
 
+cache:
+  paths:
+    - _ccache/
+
 build-job:
   stage: build
   script:
@@ -18,6 +22,17 @@ build-job:
     paths:
       - "${CI_PROJECT_DIR}/_build/meson-logs"
 
+msys2-mingw32:
+  stage: build
+  tags:
+    - win32
+  variables:
+    MSYSTEM: "MINGW32"
+    CHERE_INVOKING: "yes"
+  script:
+    - C:\msys64\usr\bin\pacman --noconfirm -Syyuu --ask 20
+    - C:\msys64\usr\bin\bash -lc "bash -x ./.gitlab-ci/test-msys2.sh"
+
 dist-job:
   stage: build
   only: