You can find the instruction to get Zulu source code in the readme text file in the root folder of all Zulu installations. That text is repeated here:
Certain portions of this software are based on source code from OpenJDK
(http://openjdk.java.net/) and licensed under the GNU General Public License version 2 (GPLv2) with the Classpath Exception (http://openjdk.java.net/legal/gplv2+ce.html). For a period of three years from the date of your receipt of this software, Azul will provide upon request, a complete machine readable copy of the source code for such portions based on OpenJDK on a medium customarily used for software interchange for a charge no more than the cost of physically performing source distribution.
Please email email@example.com for further information. Include version code in your email.
Once you lookup your version code and email it to firstname.lastname@example.org, your request will be fulfilled. Each source snapshot is a zip file and per release takes upwards of 125MB disk, so plan on providing a place to receive the file transfer (i.e. not by email.) We don’t post source on the website.
As far as your build scripts question, we build Zulu using OpenJDK’s own build scripts. BTW 8 is so much nicer than 7 or 6, that I recommend once you get source, whether from Zulu snapshots or directly from the OpenJDK project, you study 8 first. The main script is called Makefile. I’m simplifying a bit, but ultimately we just use the existing images target as is. The heart of our build is thus simply to invoke make images, but wrapped up as a Jenkins task to set version numbers. Similar tasks are used for 7 and 6, though syntax is not as elegant.
Hope this helps. Matt