#36023 build: Always define HAVE_SYSTEM

• Silence compiler warnings on platforms lacking std::system such as iOS
• Clean up build logs for developers targeting platforms without std::system

Switches the CMake definition of HAVE_SYSTEM in bitcoin-build-config.h.in from #cmakedefine to #cmakedefine01. This ensures HAVE_SYSTEM is always defined as either 1 or 0 instead of being left undefined when std::system is absent.

Problem: When compiling on platforms lacking std::system (such as iOS), an undefined HAVE_SYSTEM triggers -Wundef compiler warnings at #if HAVE_SYSTEM check sites.

P4 · platform fix

• P4 because it only fixes minor compiler warnings on a non-primary platform like iOS
• It does not fix any build failures on supported platforms

P4 because it fixes a minor compiler warning (-Wundef) on a non-primary platform (iOS) without fixing any supported platform build failure. Author noted: 'When this feature is unavailable, #cmakedefine leaves its macro undefined, triggering -Wundef. I noticed this while building the kernel for iOS.'

Membership: Modifies cmake/bitcoin-build-config.h.in.

Factors: security/stability 0, bug 0, performance 0, user value 1, leverage 0

• Ready to review
• The patch is minimal and clean with no open blockers

The patch is minimal and clean, with no open blockers preventing review of the current commit.

Author status: active, last responded on 2026-08-19 explaining preference for #cmakedefine01 over #if defined

Open concerns:

• Reviewers questioned ad-hoc changes to header definitions and suggested either reverting call sites to use #if defined(...) or establishing a project-wide convention enforced by a linter (fanquake, hebasto, maflcko).

Resolved concerns:

• Author dropped a simultaneous modification to HAVE_FDATASYNC after feedback.

• Skeptical of ad-hoc macro changes without a project-wide convention (fanquake, hebasto, maflcko)
• Suggested reverting check sites instead of altering cmake definitions (hebasto)
• Suggested establishing and enforcing a global macro standard via linter (maflcko)

Mild skepticism over ad-hoc cmake macro style without broader conventions (fanquake, hebasto, maflcko).

Maintainers pushed back against one-off macro alterations without a unified style or lint rule, though no outright concept NACK was maintained after the author trimmed the PR.

• fanquake objected to random macro changes: 'Please don't make random changes for no reason.'
• hebasto suggested handling it at call sites: 'Maybe simply revert 976b034b13d28877aee641833d5ee28a8cc5d83f?'
• maflcko asked whether a consistent macro style should be enforced globally via a linter.

Review verdicts (DrahtBot): 0

0 lines under test/bench/ci.

• cmake/bitcoin-build-config.h.in +1/-1

Changes the CMake build configuration for HAVE_SYSTEM to use #cmakedefine01 so it is always defined as 0 or 1. This prevents -Wundef warnings when building the kernel on platforms lacking std::system, such as iOS. Initial reviewer feedback questioned making ad-hoc macro changes without a consistent style or linter. The patch is small and ready for review.