cmake: make sure check_* uses 2 different variables in CheckX11

cmake/sdlchecks.cmake

@@ -457,13 +457,13 @@ macro(CheckX11)
         set(SDL_X11_SHARED OFF)
       endif()
 
-      check_symbol_exists(shmat "sys/shm.h" HAVE_SHMAT)
-      if(NOT HAVE_SHMAT)
-        check_library_exists(ipc shmat "" HAVE_SHMAT)
-        if(HAVE_SHMAT)
+      check_symbol_exists(shmat "sys/shm.h" HAVE_SHMAT_IN_LIBC)
+      if(NOT HAVE_SHMAT_IN_LIBC)
+        check_library_exists(ipc shmat "" HAVE_SHMAT_IN_LIBIPC)
+        if(HAVE_SHMAT_IN_LIBIPC)
           list(APPEND EXTRA_LIBS ipc)
         endif()
-        if(NOT HAVE_SHMAT)
+        if(NOT HAVE_SHMAT_IN_LIBIPC)
           list(APPEND EXTRA_CFLAGS "-DNO_SHARED_MEMORY")
         endif()
       endif()