Consumer-side fallout from #965 (WSC): unconditional Linux deps, missing Apple toggle, silent submodule failures

Tracking issue for consumer-side fallout from #965 (WebSocket Compression support). Even with compression turned off, consumers must take on new dependencies and submodules. Filing here so the work can be scoped/split.

Gaps identified

• (fixed by companion PR) Linux CMakeLists.txt:84-86 adds websocketpp_websocket.cpp/.h to LINUX_SOURCE_FILES unconditionally. Consumers building with HC_NOWEBSOCKETS still compile the TU and require External/asio + External/websocketpp submodules to be initialized.
• (fixed by companion PR) Build/libHttpClient.Linux/install_dependencies.bash installs zlib1g / zlib1g-dev unconditionally. Should honor HC_NOZLIB and HC_NOWEBSOCKETS.
• Apple has no HC_ENABLE_WEBSOCKET_COMPRESSION toggle. The macro is hard-coded =1 in 4 places in Build/libHttpClient.Apple.C/libHttpClient.xcodeproj/project.pbxproj (iOS/macOS Debug/Release). Other HC_* knobs are exposed as build settings; this one should follow suit.
• Win32 / GDK silent feature loss when boost-wintls submodule is missing. All WSC-related per-file ClCompile entries in Win32.Shared.vcxitems / GDK.Shared.vcxitems are guarded with Exists('$(HCRoot)\External\boost-wintls\include\wintls.hpp') AND HCEnableWebSocketCompression == 'true'. A consumer who forgets git submodule update --init --recursive gets a working build that silently lacks compression. Suggest emitting an MSBuild <Error> (or at least <Warning>) when HCEnableWebSocketCompression=true and the submodule file is absent.
• %(PreprocessorDefinitions) inheritance bug sweep. #988 fixes one instance (ASIO_DISABLE_SERIAL_PORT was dropped on the per-file websocketpp_websocket.cpp overrides because shared-items %(PreprocessorDefinitions) doesn't inherit from the consuming project's ItemDefinitionGroup). The same pattern may exist for other per-file overrides added in #965 and should be audited.

Why "turn it off" doesn't free consumers

Companion PR

A companion PR addresses the first two checklist items. Filing the others here because they involve broader design decisions (Apple build settings; MSBuild error emission; per-file-defines audit) that may warrant author input.

Commencez par répartir le travail restant entre CMakeLists.txt, install_dependencies.bash, le Apple project.pbxproj, Win32.Shared.vcxitems, GDK.Shared.vcxitems et les définitions du préprocesseur par fichier de #988. Examinez d’abord les modifications complémentaires existantes et les gardes de compilation indiqués. Le travail est terminé lorsque les builds avec la compression désactivée évitent les dépendances inutiles, qu’Apple expose le basculement, que les sous-modules Win32/GDK manquants sont signalés et que l’audit des remplacements est terminé.