Limitations:
currently the tox core library does not ship proper public headers,
so we'll have to tell the configure script where the headers are (i.e.
directory in the ProjectTox-Core source tree.
Since these headers are not really public, they include sodium.h, so
right now the toxic configure script will check for sodium.h as well.
We also need to look for the libsodium library for linking vs Tox core.
