The symbol must be defined before you #include the header, not when you use the code.
Therefore, within a file there is generally no way to have some code that "uses" the symbol and some that does not.
However, you can #define a symbol before #including the header in some files and not others.
// _PSTAT64 space
Of course if you then pass data types that are altered on the basis of _PSTAT64 between files you will run into trouble.
// non _PSTAT64 space