On Windows the KWSys System package generates escapes for command-line
arguments.  This fix enables quoting of the empty string as an argument.
This also adds a test to pass an empty argument to a custom command.

BUG: Fix kwsysSystem_Shell_GetArgumentForWindows to reset the windows trailing backslash count to zero when a make variable reference is encountered.

ENH: Improved escaping in kwsys/System.  Added escape of % for NMake.  Added escape of ; for the VS IDE.

ENH: Added more special unix shell characters that require quoting.  Added escaping of % as %% for shells inside mingw32-make.

ENH: Adding Shell_Flag_EchoWindows option to setup escapes for arguments to the native echo command in a shell.  This special case is needed to avoid adding quotes when passing text to echo in a native windows shell which does no command line parsing at all.

ENH: Re-implemented command line argument shell quoting to support several platforms with one code base.

BUG: Windows_ShellArgument: need to escape if the string contains one of a set of special characters as well as spaces.  Moved test for needing escapes to a separate method kwsysSystemWindowsShellArgumentNeedsEscape.

ENH: Adding 'System' component of C sources to hold system tools written in C.  Moved windows shell command line argument escaping code to kwsysSystem_Windows_ShellArgument and kwsysSystem_Windows_ShellArgumentSize.