chore(cmake): carry option-driven defines on pybind11_headers INTERFACE

[henryiii]

This is a general fix anyway, can stand on its own.

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Description

PYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION and PYBIND11_SIMPLE_GIL_MANAGEMENT were applied with a directory-scoped add_compile_definitions, so they only took effect in the master-project (test) build. Move them onto the pybind11_headers INTERFACE, the same pattern as PYBIND11_INTERNALS_VERSION. The options now also work in add_subdirectory mode and ride the exported/installed targets.

This is step 0 of the optional pre-compilation work: must-match configuration macros have to propagate through pybind11::headers so a future precompiled static library and consumer modules always agree.

Suggested changelog entry:

• The CMake options PYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION and PYBIND11_SIMPLE_GIL_MANAGEMENT now apply in add_subdirectory mode and to installed/exported targets, not only when pybind11 is the top-level project.

[rwgk]

Reviewed with codex gpt-5.6-sol ultra:

This change looks right to me: these configuration-dependent definitions belong on pybind11_headers as usage requirements, so they propagate through the target graph and survive export/install.

I'm approving, but I would strongly suggest adding regression coverage before merging. We already have focused CMake integration tests under tests/test_cmake_build, including small add_subdirectory and installed-target consumers, so this should fit naturally into the existing structure.

The current option-enabled CI configurations do not distinguish the old behavior from the new behavior: directory-scoped definitions already reached pybind11’s top-level test targets, while the separately configured consumer tests neither forward these options nor assert the resulting macros.

I think the existing subdirectory_target and installed_target tests could be extended by:

• forwarding the two enabled option values into the nested configurations;
• defining test-only “expect this macro” flags on the consumer targets; and
• adding conditional #ifndef checks to their shared main.cpp.

Existing CI jobs already enable each option and run test_cmake_build, so this should not require new workflow jobs. The resulting tests should fail on master and pass with this PR.