declare z85_encode / decode in zmq_utils

they are used by curve_keygen, but not available
include missing platform.hpp in curve_keygen

it was excluded, so HAVE_SODIUM would never be defined,
leading to curve_keygen always reporting "recompile with libsodium"
Merge pull request #689 from minrk/curve_keygen

include missing platform.hpp in curve_keygen
config for libsodium in alternate path, using CPP flags for both C and C++ sources.