Generate compile_commands.json automatically for clangd with CMake

CMakeLists.txt

# Automatically create and update a symlink to the compile_commands.json file in
# the source directory, so that it is in a known location and can be picked up
# easily by clangd. When using Visual Studio Code, use the clangd extension,
# version 0.3.2 or higher, so the clangd language server automatically restarts
# on configuration change, which is useful when switching between Debug and
# Release builds (to switch between debugging and testing performance). See
# https://github.com/clangd/vscode-clangd/issues/300. Always build the "ALL"
# target during development, to ensure the compile_commands.json is in sync.
if(CMAKE_EXPORT_COMPILE_COMMANDS)
    add_custom_target(
        copy_compile_commands ALL
        COMMAND ${CMAKE_COMMAND} -E rm -f
                "${CMAKE_CURRENT_SOURCE_DIR}/compile_commands.json"
        COMMAND
            ${CMAKE_COMMAND} -E create_symlink
            "${CMAKE_CURRENT_BINARY_DIR}/compile_commands.json"
            "${CMAKE_CURRENT_SOURCE_DIR}/compile_commands.json")
endif()