Add qt6 usage to mxe-octave * src/qt6-qtbase-1-fixes.patch, src/qt6-qtbase.mk, src/qt6-qtimageformats-1-fixes.patch, src/qt6-qtimageformats.mk, src/qt6-qtsvg.mk, src/qt6-qttools.mk, src/qt6-qttranslations.mk, src/qt6.mk, src/qt6-qt5compat.mk: new files * Makefile.in: add use of ENABLE_QT and add qt6 values * configure.ac: add --enable-qt=xxx option, remove--enable-qt5 * dist-files.mk, index.html: add refs to new files * src/blas_switch.mk, src/gnuplot.mk, src/opengl_switch.mk, src/default-octave.mk, src/qscintilla.mk, src/release-octave.mk, src/stable-octave.mk: add prrocessing of ENABLE_QT value * src/qt5.mk: dont include qt6* files in dependancies * src/gnuplot-2-qt6.patch: new patch file * binary-dist-rules.mk: include qtversion in build of octave-launch * installer-files/octave-firsttime.vbs, installer-files/octave-launch.c installer-files/octave.bat, installer-files/octave.vbs : include qt6 file paths

--- a/installer-files/octave.vbs	Thu Aug 17 16:44:07 2023 -0400
+++ b/installer-files/octave.vbs	Wed Aug 23 08:48:00 2023 -0400
@@ -60,7 +60,10 @@
 End If
 
 ' set Qt plugin directory and path 
-If objFSO.FolderExists(OctavePath & "\qt5\bin") Then
+If objFSO.FolderExists(OctavePath & "\qt6\bin") Then
+  wshSystemEnv("PATH") = OctavePath & "\qt6\bin;" & wshSystemEnv("PATH")
+  wshSystemEnv("QT_PLUGIN_PATH") = OctavePath & "\qt6\plugins"
+ElseIf objFSO.FolderExists(OctavePath & "\qt5\bin") Then
   wshSystemEnv("PATH") = OctavePath & "\qt5\bin;" & wshSystemEnv("PATH")
   wshSystemEnv("QT_PLUGIN_PATH") = OctavePath & "\qt5\plugins"
 Else