[PATCH] Fixes sentinel warnings on GCC4

[David Hampton]

On Mon, 2005-12-05 at 21:25 -0500, Chris Shoemaker wrote:

> The #define NULL 0 case is explicitly allowed by the ISO C standard,
> and it's easy to write portable code that works for that case, so why
> wouldn't we?

I don't know for sure, but I strongly suspect that our code is already
limited to compiling with the gnu tool chain.  Given that gcc seems to
do the right thing where NULL is concerned, I feel no need to do
anything more than add a note that gcc is required to compile gnucash.
I've got too many other dragons to slay.

David