fix error in zsh environment

When I 'source SOURCEME' in zsh environment,  Zsh doesn't recoginze variable BASH_SOURCE. So I write some judgment statements to fix it.
Hope you can accept this pull request.
I am not aware of zsh, but how to let the user know whether he/she is sourcing properly or not.

Also if you think this check not at all necessary. then please delete code instead of commenting out. 

Thanks