Modify add_xcode_base_configuration to respect existing base configurations

Original PR at https://github.com/CocoaPods/CocoaPods/pull/2146.
Closes https://github.com/CocoaPods/CocoaPods/issues/1736.

 - Do not change the base configuration when it is already set
 - Notify the user when there is an existing base configuration

Change string quoting from double to single quotes.

Split lines longer than 80 characters into multiple lines.

Quote initial string fragment with single quotes rather than double quotes.
I would make this description a little bit more informative to other readers (and future you), e.g. “if the user has already set a config of their own”.