You need to allocate more ram to SpecialSource.jar in BuildTools.jar

BuildTools Runs

"java -jar BuildData/bin/SpecialSource.jar -i work/minecraft_server.1.8.jar -m BuildData/mappings/bukkit-1.8-cl.csrg -o work/mapped.8eb82bde.jar-cl"

Instead of:

"java -Xmx3G -jar BuildData/bin/SpecialSource.jar -i work/minecraft_server.1.8.jar -m BuildData/mappings/bukkit-1.8-cl.csrg -o work/mapped.8eb82bde.jar-cl"

This issue causes the JVM to run out of memory and crashing making it impossible to build spigot