meson: Warn when git is missing in non-dist builds

8b5060349a fixes the issue of version headers
not being available when building out of VCS tarball (without .git directory)
but we were getting `unknown (unsupported)` version still. Turns out the version
silently fell back to `unknown` when git was not available.

Let's warn the user when this is the case.

meson.build

@@ -1456,15 +1456,19 @@ if is_git_repository or not has_version_h
     replace_string: '@GIMP_GIT_LAST_COMMIT_YEAR@',
     fallback: 'unknown (unsupported)',
   )
-  if not is_git_repository
+
+  git = find_program('git', required: false)
+  if not is_git_repository or not git.found()
   # We create git-version.h but know it will be useless because we are
   # not in a git repository. Output a warning.
   git_warning = '''
 
         UNSUPPORTED BUILD!
 
-        This is not a distribution tarball (git-version.h missing) nor is it
-        a git repository. Therefore we have no reference for debugging.
+        This is not a distribution tarball (git-version.h missing) and
+        we could not establish the corresponding commit (either this is
+        not a git repository or git command is missing). Therefore
+        we have no reference for debugging.
         Please either use release tarballs or build from the repository.
   '''
   warning(git_warning)