meson, build: verify for up-to-date gimp-data and warn otherwise.

build/meson/check-gimp-data.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+cd $MESON_SOURCE_ROOT
+changes=$(git status --untracked-files=no --short --ignore-submodules=dirty gimp-data|wc -l)
+exit $changes

meson.build

@@ -1730,6 +1730,16 @@ if is_git_repository
   if not has_gimp_data
     error('gimp-data submodule not present. Run: git submodule update --init')
   endif
+
+  if git.found()
+    git_submodule_check = run_command(meson.project_source_root() / 'build/meson/check-gimp-data.sh',
+                                      check: false)
+    if git_submodule_check.returncode() != 0
+      submodule_warning = 'gimp-data submodule seems outdated. Possibly run from the source repository: git submodule update'
+      warning(submodule_warning)
+      warnings += submodule_warning
+    endif
+  endif
 endif
 
 configure_file(