hevea 2.18

Also: add test, fix prefix.
OK, so looking at the Makefile, the install target runs an `install.sh` script, which sources a `config.sh` script which contains the various variables (`LIBDIR`, etc.). The problem is that `config.sh` is generated during the call to `make`, where `PREFIX` isn't set. So we just need to pass `PREFIX` to both calls.