fix: make C test builds reliable across platforms

[ping-ke]

Summary

Improve the reliability and portability of the C test build workflow across Linux, macOS, WSL, and multi-config CMake generators.

Changes

• Improve cross-platform C test builds.
• Fix CMake, Boost, and benchmark handling.
• Improve build stamp and artifact validation.
• Fix C++17 test compatibility.
• Update build documentation.

Behavior Changes

Missing Boost filesystem, system, or unit-test components now cause CMake configuration to fail explicitly. Previously, the C test target could be omitted while the overall build still appeared successful.

This PR does not change:

• libethash algorithms or C source code
• the libethash ABI
• the pyethash API or package version
• Go or Python dependency versions

Validation

Recommended full validation:

./build.sh --clean
make test-all