I needed to figure out how to create a custom build yesterday (for the first time) and noticed it was down also. I needed to remove/add some libs to MDBT42Q.
But what I found is that it is real easy to do these builds, @Gordon has made the process real easy to make the build process work at https://github.com/espruino/Espruino/blob/master/README_Building.md
Took me hour or so to create a custom MDBT42Q build without any prior knowledge - I just used the Ubuntu you can fire up in Windows.
Just for anyone who had not had a go at this.