(Fw32_get_clipboard_data): Compare data on clipboard with saved copy
of what Emacs last put there. If they are the same, do not use the
clipboard copy to avoid losing data due to coding conversions.
(syms_of_w32select): DEFVAR_LISP it.
(Fw32_set_clipboard_data): Use Vnext_selection_coding_system if
non-nil. Always convert multibyte strings.
(Fw32_get_clipboard_data): Use Vnext_selection_coding_system if
non-nil. Always convert a string that includes non-ASCII characters.
(Vclipboard_coding_system): New variable.
(syms_of_w32select): DEF_VAR it.
(Fw32_set_clipboard_data): Encode string using
Vclipboard_coding_system if necessary.
(Fw32_get_clipboard_data): Decode clipboard contents using
Vclipboard_coding_system if necessary.
into Windows 95, Windows NT, and W32, respectively.
Expand "win" substring in variables referring to Microsoft Windows
constructs into "windows".
Canonicalize header comments to use same terminology.