GROOVY-6964: Build on Windows

Building on Windows was not supported because JarJarTask does not release a file lock.
Please read http://groovy.329449.n5.nabble.com/Build-issue-with-groovy-core-master-td5719619.html for the detail.
I fixed this problem.
I checked this is running correctly on Windows 8.1 and Ubuntu 14.04.