Allow use of member variable depending on preprocessor directives

Consider the following class:

class Foo 
{
public:
    #ifdef CONDITION
    int x = 0;
    #endif

    int y;

    int foo() { 
        #ifdef CONDITION
        return ++x;
        #else
        return 0;
        #endif
    }
}

int x only exists when I define CONDITION - either through a #define CONDITION or as a preprocessor definition (-D CONDITION)

This has the neat advantage that I can't compile it I use x by mistake somewhere when CONDITION isn't defined.

For example: If, by mistake, I write something like:

Foo f;
f.x = 10;

This will not be allowed to compile when I'm missing -D CONDITION

However, we get all sorts of nasty problems when class Foo is declared in a header that is used by multiple projects, where preprocessor definitions differ:
The offset of y within Foo will be different, resulting in different interpretations of how an object of Foo looks.

The question:
Is there some way in which I can declare x for anyone using Foo, but still get some sort of compiler warning/error when I try to use it without defining CONDITION?