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++)
freerds: add target exporting
cmake: install headers
Merge remote-tracking branch 'bmiklautz/session-manager' into Devel

Conflicts:
	freerds/icp/gen-c/ICP.pb-c.c
	freerds/icp/gen-c/ICP.pb-c.h
	session-manager/gen-cpp/ICP.pb.cc
	session-manager/gen-cpp/ICP.pb.h
Merge remote-tracking branch 'origin/master' into Devel
removed freerds binary
Worked on module interface, sessionmanager and PropertyManager
icp: use defined call type instead of constatn
use enum instead of constants
session-manager/icp: server side ping call added
common/call: refactored call names
call ping: fixed file headers