Building on Debian Jessie amd64, getting this:
[proguard] Reading library jar [/home/username/svn-soft/multibit/src/main/skeleton/mac/MultiBit.app/Contents/Plugins/jdk1.7.0_25.jdk/Contents/Home/jre/lib/rt.jar]
[proguard] Error: Can't read [/home/username/svn-soft/multibit/src/main/skeleton/mac/MultiBit.app/Contents/Plugins/jdk1.7.0_25.jdk/Contents/Home/jre/lib/rt.jar] (No such file or directory)
The directory is named "PlugIns" with capital "i". If you're building on Windows then it doesn't matter I guess. However, on Linux it's a huge difference. After renaming that directory to "Plugins" I managed to package multibit.