[gtkmm] Remove some obsolete files

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 0da209e447cc721b83fe7953e6b49686bfe5a92c
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Sep 2 14:02:16 2020 +0200

    Remove some obsolete files
    
    * build/reduced.m4: Removed. Not needed now when gtkmm does
    not depend on atkmm.
    * docs/reference/README: Removed
    * meson.build: Exclude some git-tracked files from generated
    tarballs. Works as intended only with a new enough dist-build-scripts.py
    file from mm-common-get.

 build/reduced.m4      | 17 -----------------
 docs/reference/README |  6 ------
 meson.build           |  9 +++++++++
 3 files changed, 9 insertions(+), 23 deletions(-)
---
diff --git a/meson.build b/meson.build
index d5a019b8..8e6a9701 100644
--- a/meson.build
+++ b/meson.build
@@ -269,12 +269,21 @@ if not meson.is_subproject()
     python3.path(), dist_changelog_py,
     project_source_root,
   )
+
+  # Don't distribute these files and directories.
+  dont_distribute = [
+    'docs' / 'tutorial.moved_to_gtkmm_documentation_module',
+    'examples.moved_to_gtkmm_documentation_module',
+    'HACKING',
+    'win32_installer',
+  ]
   # Add build scripts to the distribution directory, and delete .gitignore
   # files and an empty $MESON_DIST_ROOT/build/ directory.
   meson.add_dist_script(
     python3.path(), dist_build_scripts_py,
     project_source_root,
     'untracked' / 'build_scripts',
+    dont_distribute,
   )
 endif