PATCH (abandonment): Apply Nathan's macro trick

[Loren James Rittle]

> This re-expression of my posting is so elegant, I wonder if the
> names are still appropriate.  Maybe the first three names should be
> _GLIBCXX_IF_0, _GLIBCXX_IF_1, and _GLIBCXX_IF, and should appear in
> a more generic header.

Sorry Nathan.  You had a great idea in both theory and practice; and I
too worked on tweaking it for a while before seeing that it doesn't
really work in the exact context we have.  I have no intention on
changing the context (details below).

OK, if we had a collection of macros with known value 0 or 1 (as you
displayed when you posted the technique), then your technique works as
advertised.  The third state (of no defined value at all), in fact
can't be supported by any means I see.  Unfortunately, our
configuration currently defines a macro to 1 if a feature is available
or leaves it completely undefined if not available (this is standard
autoconf-style AFAIK).

Of course, we all know that `#if XXX' handles XXX with value 0 or not
defined in an identical manner.  Inside a macro expansion, it
obviously works quite differently.  Zack is the CPP expert and he said
it (where it was logically ``#if X \ Y \ #endif'' defined in a macro)
can't be done.  I now believe him for the general case that we have. ;-)

Regards,
Loren