First try to use pkg-config to find libtoxcore, fall back to manual
checks if pkg-config is not available.
Improve ncurses search, should now find everything on OSX.
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.
