To prevent top-level linking of all libraries.
Problem: ```pkg-config --libs toxcore``` returns all libraries that are
required by all libtox*.so libraries. This is wrong because for a
dynamically linked executable only top-level libraries need to be
supplied. ```pkg-config --libs --static toxcore``` should return all
libraries for the statically linked executable.
For example, the ToxBot https://github.com/JFreegman/ToxBot executable
uses pkg-config and is linked with the opus library, which is wrong.
Based on #533.
This library contains all the code for the old libtoxcore, libtoxav,
libtoxdns, and libtoxencryptsave. The build for toxav is still optional,
and disabling it causes libtoxcore to simply not contain those symbols
and the pkg-config file to not include opus and vpx as dependencies.
This allows us to more clearly define interfaces between modules, and have the
linker help us ensure that module boundaries are respected.
The onion/tcp/net_crypto layer is a bit too large. This is due to a cyclic
dependency (onion -> net_crypto -> TCP -> onion). We may or may not want to
break that cycle in the future to allow the onion library to exist on its own
without net_crypto.
