use AC_DEFINE instead of hardcoding preprocessor flags
use AC_DEFINE instead of hardcoding preprocessor flags

Now that all source files include config.h, we can use AC_DEFINE
to enable/disable optional features (opengl, cegui). This is
cleaner than directly appending -Dxxx directives to the preprocessor
flags. This also guarantees that everything will be appropriately
rebuilt when using the same source tree to build first with opengl
support and then without (the config.h timestamp will change,
triggering a rebuild of the files including it)

alon/spice-server,ya-mouse/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,ya-mouse/spice,mathslinux/spice,Fantu/spice,mathslinux/spice,SPICE/spice,fgouget/spice,alon/spice-server,Fantu/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,dezelin/spice,elmarco/spice,SPICE/spice,dezelin/spice,dezelin/spice,dezelin/spice,fgouget/spice,elmarco/spice,mathslinux/spice,ya-mouse/spice,fgouget/spice,mathslinux/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,alon/spice-server,alon/spice-server,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,dezelin/spice,SPICE/spice,fgouget/spice,elmarco/spice