make it possible to override desktop detection using gtk.desktop system option

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45786 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/sysopt.tex

@@ -44,6 +44,8 @@ level windows is currently supported, however this may sometimes fail and this
 option allows to override the automatic detection. Setting it to $1$ makes the
 transparency be always available (setting it can still fail, of course) and
 setting it to $0$ makes it always unavailable.}
+\twocolitem{gtk.desktop}{This option can be set to override the default desktop
+environment determination. Supported values are \texttt{GNOME} and \texttt{KDE}.}
 \end{twocollist}
 
 \wxheading{Mac}