configure: don't warn on old sdl/gtk versions if disabled.
keymap + gtk fixes.

# gpg: Signature made Tue 10 Apr 2018 10:23:37 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/ui-20180410-pull-request:
  configure: don't warn SDL abi if disabled
  configure: don't warn GTK if disabled
  gtk: drop pointless code from gd_window_close
  ui: fix keymap detection under Xwayland
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>