fix check for XRRNotifyEvent

use CheckTypeSize instead of CheckSymbolExists

From http://www.cmake.org/cmake/help/v3.0/module/CheckSymbolExists.html :
    If the symbol is a type or enum value it will not be recognized
    (consider using CheckTypeSize or CheckCSourceCompiles).

Also fixes issue4036 (https://github.com/synergy/synergy/issues/4036).
The code was there, the problem was just that someone fucked the cmake check up