BUG 14109 VTK_INSTALL_QT_DIR is only one that needs leading slash

See http://www.vtk.org/Bug/view.php?id=14109

Change-Id: I829b73500518abbab794fe7744ee5d2c8d37c36b
Merge topic '0014109-standardize-install-qt-dir' into master

23bd3f24 BUG 14109 VTK_INSTALL_QT_DIR is only one that needs leading slash