vm/master.hpp: optional reproducible VM compilation

vm/master.hpp: optional reproducible VM compilation

The old behaviour of building with `__DATE__` and
    `__TIME__` is still the default, but now it's
    possible to use `-DREPRODUCIBLE=1` and the
    `REPRODUCIBLE=1` parameter to `make` and `nmake`.

Specifically, this replaces the value of
    `FACTOR_COMPILE_TIME` with the string constant
    `"[reproducible]"`. This is purely a cosmetic
    change.