[gtk/gtk-3-24] (14 commits) ...Merge branch 'ci-gtk3-update' into 'gtk-3-24'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  14dba95... ci: Build both static and shared libraries at once (*)
  8685538... ci: Backport the changes to the Docker script (*)
  e5be27f... ci: Add a Debian-based image (*)
  b68dde9... ci: Add a Debian build (*)
  f699a29... ci: Build API reference separately (*)
  7687f9a... docs: Remove redundant gtk-doc stanza (*)
  7c23119... Use ifdef when checking G_ENABLE_DEBUG (*)
  c08908b... Fix build when G_ENABLE_DEBUG is undefined (*)
  1b255dd... Handle a missing enumeration value (*)
  db1be15... Silence compiler warnings in release builds (*)
  b4808c1... Silence a compiler warning (*)
  aac214f... testsuite/gtk: Add xfail tests (*)
  41330b6... testsuite: Handle icontheme test better (*)
  8b5c8a2... Merge branch 'ci-gtk3-update' into 'gtk-3-24'

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