Additions to 4.x conversion

As explained here (https://github.com/jib/libvmod-statsd/pull/11), I've made a PR for 4.x conversion before seeing yours.

Both PR were very close. Yours is better at code level. Here are some additions I suggest for your PR (hoping it will be merged soon ;o)):

- Cleaning of generated RST files with `make clean`
- Removal of `VARNISHSRC` and notification about 4.x compatibility in the README