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.
Merge pull request #690 from mattconnolly/master

config for libsodium in alternate path, using CPP flags for both C and C...
add zmq_curve_keypair to zmq_utils
use zmq_curve_keypair in curve_keygen
return NULL and set EINVAL on bad z85 input

asserts aren't appropriate for checking user input.