Serialize creation of source tarballs to prevent git lockfile collisions
When these targets are run in parallel on git version 2.17, the result is this:
Loading source tree from HEAD...
error: could not lock config file .git/config: File exists
fatal: failed to register url for submodule path 'ThirdParty/IceT/vtkicet'
Forcing them to serial for now.
Edited by Shawn Waldon