How to separate your code from specific customer code?

I have the following design problem:

I have many lines of object oriented source code (C++) and our customers want specific changes to our code to fit their needs. Here a very simplified example:

void somefunction() {
    // do something
}

The function after I inserted the customer wishes:

void somefunction() {
    // do something
    setFlag(5000);
}

This looks not so bad, but we have many customers which want to set their own flag values on many different locations in the code. The code is getting more and more messy. How can I separate these customer code from my source code? Is there any design pattern?