[atkmm] Meson build: Fix versioning on macOS

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 4e30c2f8fd2004076dcd069a06f632f02bbfa901
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Thu Oct 1 09:57:43 2020 +0200

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

 atk/atkmm/meson.build | 5 ++++-
 meson.build           | 7 ++++++-
 2 files changed, 10 insertions(+), 2 deletions(-)
---
diff --git a/atk/atkmm/meson.build b/atk/atkmm/meson.build
index d393cfa..cb3db32 100644
--- a/atk/atkmm/meson.build
+++ b/atk/atkmm/meson.build
@@ -2,7 +2,8 @@
 
 # Input: atkmm_build_dep, atkmm_pcname, maintainer_mode, project_source_root,
 #        generate_binding_py, m4_files, atkmm_libversion, install_includedir,
-#        python3, atkmm_rc, dummy_header_py, gmmproc_dir, msvc14x_toolset_ver
+#        python3, atkmm_rc, dummy_header_py, gmmproc_dir, msvc14x_toolset_ver,
+#        macos_darwin_versions
 # Output: hg_ccg_basenames, extra_h_files, built_h_file_targets, built_files_root,
 #         atkmm_dep
 
@@ -150,6 +151,7 @@ if maintainer_mode
   atkmm_library = library(atkmm_libname, extra_atkmm_objects,
     built_cc_file_targets, extra_cc_files, built_dummy_h_file_target,
     version: atkmm_libversion,
+    darwin_versions: macos_darwin_versions,
     include_directories: extra_include_dirs,
     cpp_args: atkmm_cpp_args,
     dependencies: atkmm_build_dep,
@@ -193,6 +195,7 @@ else # not maintainer_mode
   atkmm_library = library(atkmm_libname, extra_atkmm_objects,
     built_cc_files, extra_cc_files,
     version: atkmm_libversion,
+    darwin_versions: macos_darwin_versions,
     include_directories: extra_include_dirs,
     cpp_args: atkmm_cpp_args,
     dependencies: atkmm_build_dep,
diff --git a/meson.build b/meson.build
index bf74ca6..ccfcac3 100644
--- a/meson.build
+++ b/meson.build
@@ -29,7 +29,12 @@ libtool_soversion = [2, 0, 1]
 atkmm_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.