Use shorter root path in script building Slicer trunk

This is an attempt to address the "too much argument" error passed
to the meta object compiler on Linux.

For example, for the build directory of Slicer:

* Before:

  /home/kitware/Dashboards/Nightly/Slicer4
  /home/kitware/Dashboards/Nightly/Slicer-build-64bits-QT4.8.5-PythonQt-With-Tcl-CLI-Release-nightly

* After:

  /home/kitware/Dashboards/Nightly/Slicer-0
  /home/kitware/Dashboards/Nightly/Slicer-0-build
  /home/kitware/Dashboards/Nightly/Slicer-0-build - 64bits-QT4.8.5-NoVTKDebugLeaks-Doxygen-Release-Nightly.tx

For sake of consistency, scripts for windows, MacOSx and Linux have
been updated.