Both the source tarball and the source zip file are incomplete. They are missing several file/dirs (e.g. the antlib dir is missing) which prevent building jruby:
bash-4.2$ tar xvf jruby-src-1.7.0.preview2.tar.gz
bash-4.2$ cd jruby-1.7.0.preview2/
/private/tmp/jruby/jruby-1.7.0.preview2/build.xml:33: Cannot find /private/tmp/jruby/jruby-1.7.0.preview2/antlib/test.xml imported from /private/tmp/jruby/jruby-1.7.0.preview2/build.xml
Total time: 0 seconds
However, they are included in the source tarball/zip from github (tag 1.7.0.preview2).