Guard PIConnection, thread primitives, PICodeParser and other feature code behind PIP_HAS_THREADS/PIP_HAS_FILESYSTEM. In CMake generate the feature flags summary from the single PIP_HAS_FLAGS list (used by the add_definitions loop as well), default ICU to OFF and make introspection also require PIP_HAS_SOCKET. Build system/thread test suites only when the corresponding feature flags are enabled.