Gnuplot 5.4.9 release tarball

[Ethan A M. <me...@uw...>]

I have placed a release tarball of 5.4.9 on SourceForge

    https://sourceforge.net/projects/gnuplot/files/gnuplot/5.4.9/gnuplot-5.4.9.tar.gz

This is probably the final update to version 5.4,
as a full release of version 6 is expected by the end of this year.

In addition to a few bug fixes, it includes a change to the configuration system
to support linking against Qt6 (previous releases checked only for Qt5 or Qt4).  

Changes in 5.4.9
================
* NEW    qt:  support building with Qt6
* CHANGE check plugin version at time of import                         Bug #2629
* FIX    prevent segfault if GNUPLOT_DRIVER_DIR is invalid
* FIX    (regression) only y autoscale yerrorbars to points in xrange   Bug #2631
* FIX    possible memory corruption if clipping causes 0-size polygons
* FIX    wxt: initialization of key box toggle state
* FIX    post: handle pixmaps                                           Bug #2644


    Ethan