COMP: ensure copied items writable in cpack staging dir
Should fix errors like below when packaging with Homebrew Qt.
error: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/install_name_tool: can't open input file: /Users/inorton/git/slcr/r4nj/Slicer-build/_CPack_Packages/macosx-amd64/DragNDrop/Slicer-4.9.0-2016-10-03-macosx-amd64/Slicer.app/Contents/Frameworks/QtTest.framework/Versions/4/QtTest for writing (Permission denied)