cmake: removed an add_definitions call.

cmake/sdlchecks.cmake

@@ -442,7 +442,6 @@ macro(CheckX11)
           list(APPEND EXTRA_LIBS ipc)
         endif()
         if(NOT HAVE_SHMAT)
-          add_definitions(-DNO_SHARED_MEMORY)
           list(APPEND EXTRA_CFLAGS "-DNO_SHARED_MEMORY")
         endif()
       endif()