build/windows: Fail on warnings on Clang-CL builds for babl sake

Babl can now be built cleanly on MSVC, yay!

.gitlab-ci.yml

@@ -543,6 +543,7 @@ gimp-snap:
     - if: '($GIMP_CI_MESON_MSVC != null || "$[[ inputs.test_pipeline ]]" =~ /.*GIMP_CI_MESON_MSVC.*/) && $CI_JOB_NAME !~ /.*installer.*/ && $CI_JOB_NAME !~ /.*store.*/ && $CI_JOB_NAME !~ /.*eol.*/ && $CI_JOB_NAME !~ /.*aarch64.*/'
       variables:
         VCPKG_ROOT: "$CI_PROJECT_DIR/vcpkg"
+        WARN_AS_ERROR_ON_CI: '-Dwerror=true'
         VARIANT: "-msvc"
     - <<: *CI_RELEASE
       variables:

build/windows/1_build-deps-msys2.ps1

@@ -164,7 +164,7 @@ function self_build ([string]$repo, [array]$branch, [array]$patches, [array]$opt
 
 if ($env:VCPKG_ROOT)
   {
-    self_build https://gitlab.gnome.org/GNOME/babl @('-Denable-gir=false')
+    self_build https://gitlab.gnome.org/GNOME/babl @('-Denable-gir=false', "$WARN_AS_ERROR_ON_CI")
     self_build https://gitlab.gnome.org/GNOME/gegl @('build\windows\patches\0001-libs-operations-meson-Do-not-build-CTX-which-is-Unix.patch') @('-Dintrospection=false')
     exit 0
   }