[gtkmm] Meson build: Fix versioning on macOS

[Kjell Ahlstedt <kjellahl src gnome org>]

commit c0e99cf52a184aa99bd9d3001be5242a39870b97
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Thu Oct 1 11:18:56 2020 +0200

    Meson build: Fix versioning on macOS
    
    See libsigcplusplus, pull request 65

 gtk/gtkmm/meson.build | 3 ++-
 meson.build           | 7 ++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkmm/meson.build b/gtk/gtkmm/meson.build
index 80c8d2ef..25a3feda 100644
--- a/gtk/gtkmm/meson.build
+++ b/gtk/gtkmm/meson.build
@@ -2,7 +2,7 @@
 
 # Input: gtkmm_build_dep, gtkmm_pcname, maintainer_mode, project_source_root,
 #        generate_binding_py, m4_files, gtkmm_libversion, install_includedir,
-#        python3, gtkmm_rc, dummy_header_py, gmmproc_dir
+#        python3, gtkmm_rc, dummy_header_py, gmmproc_dir, macos_darwin_versions,
 #        build_deprecated_api, gmmproc_extra_m4_dirs, is_host_windows,
 #        gdkmm_library, gdkmm_used_built_h_file_targets, gtkmm_libname
 # Output: gtkmm_hg_ccg_basenames, gtkmm_extra_h_files, built_files_root,
@@ -490,6 +490,7 @@ gtkmm_library = library(gtkmm_libname, extra_gtkmm_objects,
   include_directories: extra_include_dirs,
   cpp_args: gtkmm_cpp_args,
   version: gtkmm_libversion,
+  darwin_versions: macos_darwin_versions,
   dependencies: gtkmm_build_dep,
   link_whole: gdkmm_library,
   install: true,
diff --git a/meson.build b/meson.build
index 8e6a9701..6798b777 100644
--- a/meson.build
+++ b/meson.build
@@ -30,7 +30,12 @@ libtool_soversion = [0, 0, 0]
 gtkmm_libversion = '@0@.@1@.@2@'.format(
   libtool_soversion[0] - libtool_soversion[2],
   libtool_soversion[2],
-  libtool_soversion[1])
+  libtool_soversion[1]
+)
+macos_darwin_versions = [
+  libtool_soversion[0] + 1,
+  '@0@.@1@'.format(libtool_soversion[0] + 1, libtool_soversion[1])
+]
 
 # Use these instead of meson.source_root() and meson.build_root() in subdirectories.
 # source_root() and build_root() are not useful, if this is a subproject.