Add support for __VA_ARGS__ in a `define MACRO

[zaun]

I'd like to create some more flexible macros, mostly for logging. Something like:

`define LOG(CATEGORY, MESSAGE, ...) \
    $display("[%s] %0t: " MESSAGE, CATEGORY, $time, __VA_ARGS__)

but at the moment this isn't supported an I get

error: malformed `define directive ('.').

This is just an enhancement request, not a bug.