So I didn't need to compile XMLTV to get it to work - simply used:
sudo apt-get install xmltv
There is a Jessie Build:
https://packages.debian.org/jessie/xmltv
This is a slightly different version to the one your trying to build, but I'm using 0.5.63 so I'm sure that shouldn't be a problem.
Probably need to make sure we get xmltv installed first before anything else, but if that does work try the following.
Use this to list all the grabbers that are installed, you're looking for
tv_grab_sd_json:
tv_find_grabbers baseline manualconfig
The grabber didn't come with the package for me so I simply downloaded the latest version from here: https://github.com/kgroeneveld/tv_grab_sd_json then copied the file to /usr/bin/. If you have the same problem and need to copy the file over make sure the permissions are the same as the other grabber files (Majority of the problems I have with Linux are permissions based, once you know that they can be fairly easy to fix: https://help.ubuntu.com/community/FilePermissions)
Once you've got everything installed you can configure the grabber, I just used the steps available here: https://www.mythtv.org/wiki/Migrating_to_Schedules_Direct_in_the_UK#Configure_your_feeds_for_the_first_time
I can't remember what questions crop up during configuration but I think I used the default ones when required, post any here if you need to. My one top tip is don't download all the channels it takes longer and you probably don't want all the channels anyway.
Hopefully if all that works well you should be able to create an xml file that contains the epg data, the next step is getting the data into Tvheadend.
I struggled with this bit, but think restarting Tvheadend was the answer in the end, so following these steps should help:
# Restart Tvheadend
# Go into the
Configuration > Channel/EPG > EPG Grabber
# Under the internal grabber section you should hopefully see the option "XMLTV: Schedules Direct JSON API"
# I didn't touch the CRON multi-line settings
# All the other sections are disabled
# I think the grabber started automatically pulling the data, if that doesn't work you can change the CRON multi-line settings to run:
http://docs.tvheadend.org/webui/config_epggrab/#cron-multi-line-config-text-areas
# Then go to
Configuration > Channel/EPG > Channels
# You'll need to go through each channel and change the EPG Source to the XMLTV grabber.
# Hopefully you'll start to see the Schedules Direct data coming into the EPG
Let me know how you get on, sounds like the install is the frustrating bit, once you get that sorted the rest should be straight forward.