[RFC] USE_ and HAVE_ definitions

At present we have a little bit of a mess with the configure.in defined
  macros.

1) defined or undefined.
  For coding: #ifdef or #ifndef
  Combined form: #if defined(X) && defined(Y) ...

2) always defined.
  For coding: #if X
  Combined form: #if X && Y ...

I'm thinking the HAVE_ should be method (1) and the USE_ should be
method (2).

Do we already have a formal policy somewhere I've missed?

If we don't, Opinions on what to adopt?

Amos

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