Putting the sources on GitHub is something, but only with a very complicated build process which is not documented or even using URL's that are private/internal to Citrix.
That's why people start to push Citrix to at least release a documentation on the build process and/or exploring the difficulty to build it. See https://xen-orchestra.com/forum/topic/562/xenserver-7-3-and-... and also the Citrix thread regarding that: https://discussions.citrix.com/topic/391868-xenserver-73-ann...
So far, no answer from Citrix.