Update in-source build error message

CMakeLists.txt

@@ -37,8 +37,9 @@ project(mruby C)
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR AND NOT (MSVC_IDE OR XCODE))
   message(FATAL_ERROR
           "\nIn-source builds are not allowed as CMake would overwrite the "
-          "Makefiles distributed with mruby. Please change to the 'build' "
-          "subdirectory and run CMake from there.")
+          "Makefiles distributed with mruby. Delete any created 'CMakeFiles' "
+          "subdirectory and 'CMakeCache.txt' file from the current directory, "
+          "change to the 'build' subdirectory, and re-run CMake from there.")
 endif()
 
 if(COMMAND cmake_policy)