Add Include Guard in Sample Modules

[threeal]

This pull request resolves #21 by introducing the following changes:

• Adds the include_guard function on the top of the sample MkdirRecursive.cmake module to prevent the function from being included twice.
• Modifies the sample MyMkdirConfig.cmake config file to append the CMAKE_CURRENT_LIST_DIR variable to the CMAKE_MODULE_PATH variable only if the CMAKE_CURRENT_LIST_DIR variable is not already present in the CMAKE_MODULE_PATH variable.