Prevent macro expansion of mblen

[Robert Lipe]

> | I still don't know of any target that has a macro but not a function.
> | The targets I've known about have a macro *and* a function, and I
> | believe this implementation is in accord with the ISO C Standard;
> | offering it just as a macro isn't (I think).
> | 
> | Unless you know of some implementation that offers it only as a macro,
> | I don't see a reason to keep debating this issue.

Since I just heard lots of eyes roll my direction (I have another target
where mblen is a #define in /usr/include) I'll confirm that I do have a
global symbol for mblen in libc.a. :-)

It's some affirmation that I've been #undef-ing it in my local copy and
have observed no link failures.

RJL