Release 3.12 packaging again had a problem with compiling the packages sources from scratch because a <mkdir> was missing, see r1678322 for the fix. We had similar problems in 3.11, see bug 57468. We should create a build.xml step which extracts the resulting source package into a fresh directory and compiles/tests the resulting code to ensure that this step is not broken again in the future. Then we should add this to the CI jobs by adding this to the "jenkins" target in build.xml or using a separate target if we want to schedule this less-often, i.e. not for all different platforms.
Added a standalone build.xml which uses the poi-src-*.zip from the assemble-step an tries to compile POI with it. This is now uses in the Jenkins builds at https://builds.apache.org/view/POI/job/POI/ to verify that the source package can be built from scratch.