cmake: don't use generated files to detect a submodule

Even if Makefile for some reason was checked in in a submodule,
it is still a generated file, will be cleaned, won't be in a source
package. One cannot jump to conclusions if it doesn't exist.

storage/rocksdb/CMakeLists.txt

@@ -5,8 +5,8 @@ MACRO(SKIP_ROCKSDB_PLUGIN msg)
   RETURN()
 ENDMACRO()
 
-IF (NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/rocksdb/Makefile")
-  SKIP_ROCKSDB_PLUGIN("Missing Makefile in rocksdb directory. Try \"git submodule update\".")
+IF (NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/rocksdb/CMakeLists.txt")
+  SKIP_ROCKSDB_PLUGIN("Missing CMakeLists.txt in rocksdb directory. Try \"git submodule update\".")
 ENDIF()
 
 CHECK_LIBRARY_EXISTS(rt timer_delete "" HAVE_TIMER_DELETE)