Fix CMake options being forced in the cache

[zfergus]

Description

Replaces the CACHE ... FORCE overrides in the CMake configuration with option defaults and normal-variable shadowing, touching the option setup, the SIMD and CUDA fallbacks, and the oneTBB recipe.

• Option defaults: IPC_TOOLKIT_BUILD_TESTS now defaults to ${IPC_TOOLKIT_TOPLEVEL_PROJECT} and IPC_TOOLKIT_BUILD_PYTHON to OFF, replacing the if/else that forced both OFF in the cache.
• SIMD fallback: shadows IPC_TOOLKIT_WITH_SIMD with a normal variable when find_package(SIMD) fails.
• CUDA fallback: when IPC_TOOLKIT_WITH_CUDA is on but no CUDA compiler is found, warns and builds without CUDA instead of failing in enable_language(CUDA).
• oneTBB: sets BUILD_SHARED_LIBS as a normal variable inside onetbb_import_target(), replacing the push_variable/pop_variable macros (−25 lines).

Impact:

• A user's -DIPC_TOOLKIT_WITH_SIMD=ON is no longer overwritten. The fallback now lasts one configure, rather than keeping the option OFF in the build directory after SIMD becomes available.
• Projects that embed IPC Toolkit no longer have their cache rewritten.
• No source or API changes.

Behavior change

• A parent project can now enable IPC_TOOLKIT_BUILD_TESTS and IPC_TOOLKIT_BUILD_PYTHON when adding IPC Toolkit as a subdirectory. The defaults are unchanged (both OFF).

Why the oneTBB change is more than style

• The old CACHE PATH ... FORCE changed the type of the BUILD_SHARED_LIBS cache entry from BOOL to PATH.
• With CMP0126 NEW, it had no effect when the caller already had a normal BUILD_SHARED_LIBS: the child scope kept seeing the caller's value, so TBB_PREFER_STATIC=ON could be ignored. Reproduced with a stub subdirectory.
• The function's own scope now provides the save/restore, and oneTBB's add_subdirectory inherits the value.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves existing functionality)

How Has This Been Tested?

CMake configure checks only. No build or test run; CI covers that.

• Top-level configure with -DIPC_TOOLKIT_BUILD_TESTS=ON: oneTBB is configured as libtbb.a, and BUILD_SHARED_LIBS is not left in the cache.
• SIMD fallback, forced with -DSIMD_CXX_FLAGS=: prints the warning, IPC_TOOLKIT_WITH_SIMD stays ON in the cache, and config.hpp leaves it undefined.
• Subproject build from a parent that sets BUILD_SHARED_LIBS ON (with IPC_TOOLKIT_WITH_ABSEIL=OFF, since Abseil can't build shared on macOS): tbb is a STATIC_LIBRARY, the parent's value is unchanged, and tests and Python default to OFF.
• Not tested: the CUDA fallback (no CUDA toolchain on this machine) and a parent opting in to tests.

Test Configuration:

• OS and Version: macOS 26.6.2
• Compiler and Version: AppleClang 21.0, CMake 4.1.1

Checklist

• I have followed the project style guide
• My code follows the clang-format style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.92%. Comparing base (61ff288) to head (cfbc46a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #268      +/-   ##
==========================================
- Coverage   96.92%   96.92%   -0.01%     
==========================================
  Files         194      194              
  Lines       17349    17348       -1     
  Branches      943      943              
==========================================
- Hits        16816    16815       -1     
  Misses        533      533              

Flag	Coverage Δ
unittests	96.92% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.