Delegate all Makefile targets to ant

Embed and use ant-contrib so that it's possible to build on systems
without ant-contrib on the default classpath.

- All `make` targets (`clean`, `html`, `latexpdf` (and now `pdf`) should continue to work
- The behaviour of these targets should match the corresponding `ant` targets (since it just calls them)

This worked for me anyway on Linux, so needs testing on MacOS where ant-contrib isn't available by default.