xorg build: use prebuild xorg if available

use xorg-server in xorg source directory if available. Otherwise
try to build xorg-server in the build directory.
cmake: fix incorrect detection of protobuf-c library
freerds: fix inclusion of protocol buffer headers
freerds: regenerate protobuf stubs
Add VBlank signal support

This patch adds a signal that is sent when the framebuffer is available
for updates.
Merge pull request #8 from hardening/vblank

Add VBlank signal support
Merge pull request #7 from bmiklautz/xserver_build

xorg build: use prebuild xorg if available
freerds: added Logon/Logoff messages
automatically generate protobuf stubs (c/c++)