Where the context is a struct passed from C with pointers
to all the functions that may be called.
Instead of referencing C functions directly, wrap them
in function pointers so pure Rust unit tests can still run.
Rust is currently optional, use the --enable-rust configure
argument to enable Rust.
By default Rust will be built in release mode. If debug is enabled
then it will be built in debug mode.
On make dist, "cargo vendor" will be run to make a local copy
of Rust dependencies for the distribution archive file.
Add autoconf checks to test for the vendored source, and if it
exists setup the build to use the vendored code instead of
fetching it from the network.
Also, as Cargo requires semantic versioning, the Suricata version
had to change from 4.0dev to 4.0.0-dev.