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.
andrey