ui: build curses, gtk and sdl as modules.

# gpg: Signature made Mon 05 Mar 2018 08:48:24 GMT
# 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-20180305-pull-request:
  ui/sdl: build as module
  audio: rename CONFIG_* to CONFIG_AUDIO_*
  ui/curses: build as module
  ui/gtk: build as module
  configure: opengl doesn't depend on x11
  configure: add X11 vars to config-host.mak
  console: add ui module loading support
  console: add and use qemu_display_find_default
  egl-headless: switch over to new display registry
  curses: switch over to new display registry
  cocoa: switch over to new display registry
  sdl: switch over to new display registry
  console: add qemu display registry, add gtk

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>