Treat bundled Boost headers as system includes

[Knogle]

Motivation

• GCC 15 treats certain Boost concept-check patterns as -Werror=nonnull errors which breaks the build, so warnings from bundled Boost headers must be suppressed without touching global compiler flags.
• The previous approach using a compiler flag to disable nonnull as an error is removed in favor of marking the bundled Boost headers as system includes so the compiler will not treat Boost warnings as project errors.

Description

• Replace include_directories(${PROJECT_SOURCE_DIR}/lib/boost) with include_directories(SYSTEM ${PROJECT_SOURCE_DIR}/lib/boost) to mark the bundled Boost headers as system includes.
• Remove the prior GCC-only -Wno-error=nonnull compile option so no ad-hoc warning-suppression flags are kept in the build system.

Testing

• No automated tests or builds were run for this change.

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Codex Task