To do this as efficiently as possible, it's probably worthwhile to use a higher radix and shorter macro names. For example:

    $ cc -E - <<.
    > #define A(x) x x x x x x
    > #define B(x) A(x) A(x) A(x) A(x)
    > #define C(x) B(x) B(x) B(x) B(x)
    > C(Noooooo)
    > .