autoconf handling of --with-libsodium sets CXXFLAGS, should be CPPFLAGS

The include directory for a custom installation of libsodium
was not being picked up correctly through the use of --with-libsodium.
This resulted in a libczmq without crypto capabilities. :/
In configure.ac libzmq sets CPPFLAGS during handling of --with-libzmq
Now the handling of --with-libsodium will too.