ci: Force build of default DDXen in the default target

If any dependency that a DDX requires is missing, the build of the DDX
will be automatically disabled, and the CI check will succeed.

We want the build to fail if any of the expected DDX cannot be built in
the CI, so let's force-enable them at build time.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>

v2: Use a variable for the DDX build args (Peter Hutterer
    <peter.hutterer@who-t.net>)

Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1743>

.gitlab-ci.yml

@@ -173,18 +173,28 @@ meson:
       XTEST_DIR: /root/xts
       PIGLIT_DIR: /root/piglit
       LP_NUM_THREADS: 0
+      MESON_DDX_BUILD_ARGS: >
+        -Dxwayland=${BUILD_XWAYLAND} -Dxorg=${BUILD_XORG} -Dxephyr=${BUILD_XEPHYR} -Dxvfb=${BUILD_XVFB} -Dxnest=${BUILD_XNEST}
+      BUILD_XEPHYR: true
+      BUILD_XNEST: true
+      BUILD_XORG: true
+      BUILD_XVFB: true
+      BUILD_XWAYLAND: true
+      MESON_EXTRA_ARGS: ${MESON_DDX_BUILD_ARGS}
 
 meson-noglamor:
     extends: meson
     variables:
-        MESON_EXTRA_ARGS: >
-          -Dglamor=false
+        MESON_EXTRA_ARGS: -Dglamor=false ${MESON_DDX_BUILD_ARGS}
 
 xwayland-nolibdecor:
     extends: meson
     variables:
-        MESON_EXTRA_ARGS: >
-          -Dlibdecor=false -Dxorg=false -Dxephyr=false -Dxvfb=false -Dxnest=false
+        BUILD_XEPHYR: false
+        BUILD_XNEST: false
+        BUILD_XORG: false
+        BUILD_XVFB: false
+        MESON_EXTRA_ARGS: -Dlibdecor=false ${MESON_DDX_BUILD_ARGS}
 
 mingw-cross-build:
     extends: .common-build-and-test