sys/_types.h: Unbreak gcc build

[des]

Summary

We can't assume that <sys/cdefs.h> is in scope.

Fixes: 85ab981a8e4e ("sys/_types.h: define fallback __(u)intcap_t")

[imp]

Oh that's what you meant. Why wouldn't it be in scope?

[olce]

For the record: Clang documents __has_feature() as a function-like macro, but GCC only says it's a "special operator", was introduced for compatibility with clang and recommends against using it.

In any case, we are already testing for whether __has_feature is defined or not in sys/cdefs.h, so we know that works. The GCC folks must indeed have taken the full compatibility route, which is sensible.

[des]

In D53980#1233312, @imp wrote:

Oh that's what you meant. Why wouldn't it be in scope?

Because ever since someone¹ added tools/build/test-includes/Makefile to our tree four years ago, we test on every buildworld that most headers can be included without prerequisites, and <sys/_types.h> is included by <sys/_callout.h> which is included by <sys/callout.h> and none of those includes <sys/cdefs.h>.

¹ that someone was you

[imp]

In D53980#1233328, @des wrote:

In D53980#1233312, @imp wrote:

Oh that's what you meant. Why wouldn't it be in scope?

Because ever since someone¹ added tools/build/test-includes/Makefile to our tree four years ago, we test on every buildworld that most headers can be included without prerequisites, and <sys/_types.h> is included by <sys/_callout.h> which is included by <sys/callout.h> and none of those includes <sys/cdefs.h>.

¹ that someone was you

Oh. That problem... I could go either way on just including it or using ifdef.