Concatenated if and preprocessor

Consider this code:
if (cond1) {
...
} else if (cond2) {
...
} else if (cond3) {
...
}
I want to activate every single if with a macro preprocessor. All the combinations are possible: only the first, only the second, only the third, the first and second... and so on.
What's the best method to have a clean code that is always compiled without errors?