Changing a defined constant in a dll compilated file from external application

Question

Please, help me to figure out this. I have the following code in C (example):

#define cutoff 22;
double age;

if (age == cutoff) {
  cout << "Good morning.";
} else {
  cout << "Good evening.";
}

This code is built as a .dll file and taken from a Gauss programming language code. I wish to change #define cutoff 22, let say, to:

#define cutoff 50 #define cutoff 51 .... etc

from the Gauss code and that the dll file can incorporate these new values into the remaining code above in the defined variable "cutoff".

How can I do that? Is there another more efficient way?

Thank you!