-I/usr/include/sodium

Alternatively:

# -I /usr/local/include/sodium

For the loader you might need to adjust:

-L/usr/local/lib

djbdnscurve6 can also be installed against DJB's orginal 'nacl':
# -I PATH/nacl-20110221/build/HOSTNAME/include/ARCH
# -L PATH/nacl-20110221/build/HOSTNAME/lib/ARCH4

Here, you need to change the include path, but also the loader path
together with a symlink of libnacl.a to libsodium.a.
