[pangomm] Meson build: If not maintainer-mode, check that generate-binding.py exists

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 118e863facb49293bb980934c6fc04f8ef7f9f33
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Tue Apr 7 16:15:32 2020 +0200

    Meson build: If not maintainer-mode, check that generate-binding.py exists
    
    Trying to build with maintainer-mode=false from a tarball generated with
    Autotools will fail with a proper error message.
    Fixes #6

 meson.build | 10 ++++++++++
 1 file changed, 10 insertions(+)
---
diff --git a/meson.build b/meson.build
index 770dd85..5ab6eed 100644
--- a/meson.build
+++ b/meson.build
@@ -215,6 +215,16 @@ if maintainer_mode
   # Copy files to untracked/build_scripts and untracked/docs.
   run_command(mm_common_get, '--force', script_dir,
     project_source_root / 'untracked' / 'docs')
+else
+  cmd_py = '''
+import os
+import sys
+sys.exit(os.path.isfile("@0@"))
+'''.format(generate_binding_py)
+  file_exists = run_command(python3, '-c', cmd_py).returncode() != 0
+  if not file_exists
+    error('Missing files in untracked/. You must enable maintainer-mode.')
+  endif
 endif
 
 # pangomm's own script files.