[gtkmm] (11 commits) ...Merge branch 'use-dllexport' into 'master'

[Chun-wei Fan <fanchunwei src gnome org>]

Summary of changes:

  1ef657a... gdk/gdkmmconfig.h.[meson|in]: Add GDKMM_API (*)
  1ff126d... gtk/gtkmmconfig.h.[meson|in]: Re-organize GTKMM_API definit (*)
  8036942... gdk/src/*.hg: Mark classes and APIs with GDKMM_API (*)
  efd3e96... gdk/gdkmm/*.h: Mark classes and APIs with GDKMM_API (*)
  8358d39... class_gtkobject.m4: Add ability to decorate generated class (*)
  fbe9097... gtk/src/*.hg: Mark classes and APIs with GTKMM_API (*)
  e137e87... gtk/gtkmm/*.h: Mark classes and APIs with GTKMM_API (*)
  b7099b7... Meson: Build gtkmm directly for all builds (*)
  0f31e0d... NMake Makefiles: Stop using gendef.exe (*)
  d7ed398... Drop gendef from the sources (*)
  fb0e714... Merge branch 'use-dllexport' into 'master'

(*) This commit already existed in another branch; no separate mail sent