Having -Werror set by default causes users' builds to fail because
toxcore is not warning-free. Failing on errors is appropriate for the
development phase, e.g. when building it in a CI enviroment, but it
doesn't make much sense to fail builds for users and let them figure out
that they need to pass -DWARNINGS=OFF to make the library build.
Compiling as C++ changes nothing semantically, but ensures that we don't
break C++ compatibility while also retaining C compatibility.
C++ compatibility is useful for tooling and additional diagnostics and
analyses.
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.
