When cross-compiling to <target> from Linux, cmake might find native Linux
libconfig and decide to build tox-bootstrapd. If the target is Windows, this
will fail, as tox-bootstrapd can't be built for Windows in the first place. If
the target is Linux of some other architecture, then using host native
libconfig will fail too. Thus an option is needed to guard against this.
- This PR also adds a DEBUG cmake option that enables -DTOX_DEBUG.
- We also remove `-Wall`, because there are too many warnings, and nobody really
looks at them at the moment. We'll see about fixing them soon. We'll also want
to enable `-Werror` at some point.
- Finally, this PR enables `-O3` to make sure toxcore still works correctly
under heavy compiler optimisations.
