move zlib and microhttpd into submodules

[neodix42]

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[chatgpt-codex-connector]

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[github-actions]

• Emscripten configure will now fail: BuildMHD.cmake forces MHD_FOUND to FALSE for emscripten, but blockchain-explorer/CMakeLists.txt now hard-fails when MHD_FOUND is false, and CMakeLists.txt still unconditionally adds blockchain-explorer. This breaks the existing wasm build. Consider guarding add_subdirectory(blockchain-explorer) or skipping the fatal for USE_EMSCRIPTEN. CMake/BuildMHD.cmake:10-13, blockchain-explorer/CMakeLists.txt:14-16, CMakeLists.txt:589.
• BuildZlib.cmake unconditionally overwrites ZLIB_* cache entries with FORCE, so callers that pass their own zlib (e.g., the wasm build script’s -DZLIB_FOUND/-DZLIB_LIBRARIES) will be ignored. This can cause cross-compile zlib mismatch or build failures. Consider only setting these when ZLIB_FOUND is not already set or skip in USE_EMSCRIPTEN. CMake/BuildZlib.cmake:143-151.
• MSVC toolset is hardcoded inconsistently: libmicrohttpd build uses v143 + w32/VS2022, while zlib build uses v142. This will break VS2019 or VS2022 setups depending on what’s installed. Consider using ${CMAKE_VS_PLATFORM_TOOLSET} or omitting the toolset override. CMake/BuildMHD.cmake:14-31, CMake/BuildZlib.cmake:57-74.

[github-actions]

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