Enable command-line defines as textmacros
lg with tweaks:
masm doesn't do it, but I think we should diag on redefinitions in -D flags, at least if the redefinition has a different value. that's likely always a bug in the build system invoking us.
Can you test ifdef elseifdef too?
I'm not sure there's a clean way to do this that isn't an error, and I'd like to maintain compatibility with ml64.exe on this. If you think it's worth special-casing this to plumb a warning through, we can do that... but I'm not convinced it's a good idea.