BundleUtilities: Ensure framework symlinks and Info.plist exist

This restores Qt SDK 4.8 and OS X >= 10.6.5 codesign compatibility
improving embedding frameworks using correct bundle layout described at:

https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/FrameworkAnatomy.html

1. If Versions/VERSION/Resources/Info.plist is missing, well known
   incorrect locations are checked for Info.plist and Info.plist is
   copied from there, otherwise codesign will fail.

2. Root framework symlinks to binary and Resources are restored to
   point inside Versions/Current, otherwise Qt 4.8 looking for
   Resources/ in framework root will fail.
Help: Document CMAKE_XCODE_ATTRIBUTE_<an-attribute> variable (#15215)
FindCUDA: Remove rpath logic outdated by CMake's own @rpath support

Otherwise binaries end up with two copies of the same value and the OS X
install_name_tool may corrupt them.
FindHg: Do not use cygwin hg on Windows

The cygwin hg client is a text file with a '#!/bin/python" line.
This cannot run on Windows.
Tests/ExternalProject: Skip Windows hg tests with cygwin hg

The cygwin hg client is a text file with a '#!/bin/python" line.
This cannot run on Windows.
Merge branch 'fix-OSX-bundle-rpaths-and-Qt5' into release
Merge branch 'cuda-rpath-osx' into release
Merge branch 'FindHg-no-cygwin-hg-on-windows' into release
Merge branch 'ExternalProject-no-cygwin-hg-on-windows' into release
Merge branch 'doc-CMAKE_XCODE_ATTRIBUTE' into release