> If stdbool is not available, C99 still defines false and true as macros.

What do you mean by that? In C99, true and false are *not* defined in
a translation unit unless stdbool.h is included, see 7.16.

> If stdbool is available, then most compilers will define false and true 
> as macros.

*In* the header file, that is. C99 *requires* the implementation to
define true and false *only* in the header file, and *only* as
macros. Anything else would not be conforming to C99.

> In both cases, if stdbool was included by any system header in the 
> future, that would conflict with your current definition.

No system header should ever include stdbool.h; doing so would be
in violation of the standards.

> Do you agree with these changes then?

Not at all; I think the patch should be rejected.