diff --git a/cmake/sdlchecks.cmake b/cmake/sdlchecks.cmake
index ea2a421e6..39777452c 100644
--- a/cmake/sdlchecks.cmake
+++ b/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()