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.

Start by splitting the remaining work across CMakeLists.txt, install_dependencies.bash, the Apple project.pbxproj, Win32.Shared.vcxitems, GDK.Shared.vcxitems, and the per-file preprocessor definitions from #988. Review the existing companion changes and the listed build guards first. Done means compression-off builds avoid unnecessary dependencies, Apple exposes the toggle, missing Win32/GDK submodules are reported, and the override audit is complete.