Qt 6 and submodules

[biagas]

@griffin28 posed the question about future of Qt5, so I started looking into updating VisIt's use of Qt to version 6.

Preliminary thoughts/discoveries:

• Will need to update Qwt to version 6.2.0 (https://sourceforge.net/p/qwt/news/)
• Requires newer compilers https://doc.qt.io/qt-6/supported-platforms.html
• Qt 6 full tarball is larger, takes much longer to extract (subjectively, I didn't time it), and has a lot more options.
• The list of 'skip' this or 'no-make' that is larger to get just what VisIt needs
• Qt 6 uses CMake for build, but recommends using their script for configuring so as to pick up their default options.
• A new version of CMake must be in your PATH, otherwise there will be errors due to use of newer CMake functions. (Default cmake on LC is too old).

Can we build only the modules needed by VisIt?

Related to the increased size and number of options for building, I looked into downloading and building only the modules that VisIt needs. I started with Qt 5.14.2, since VisIt currently supports it and it would be easy to see what modules are needed.

• When building modules individually it starts with qtbase, which is built in the same manner as the full tarball (just with fewer options).
• Other modules are then built using 'qmake' from where qtbase was installed. Just qmake, make, install.
• qttools submodule was needed
• qtsvg submodule was needed (used by Qwt)
• qtx11extras was needed on Linux

With only those 4 modules, VisIt compiled successfully and simple tests worked.
Caveat: I did not test all apsects of the gui, nor did I test python gui needs (like PySide).

Building individual modules with Qt 6 is a slightly different process:

• build qtbase (configure, make, install)
• submodules configure using CMake
• at a minimum, submodules require setting Qt_DIR pointing to 'qtbase install location/lib/cmake/Qt6', and CMAKE_INSTALL_PREFIX pointing to 'qtbase install location'
• There is no qtx11extras, but there are notes about the changes

Question:

Do we want to move to an individual module build of Qt instead of using the full tarball?

If yes, which modules are needed (beyond the minimum to get VisIt compiling)?

• qtbase
• qttools
• qtsvg
• ?

Is QML needed? It didn't build with qtbase (or somehow I got the build options wrong). If not in qtbase, I am not sure which submodule contains it.

Here's qt docs on modules, thought not everything listed actually has a separate tarball.
The available submodule downloads: https://download.qt.io/archive/qt/6.4/6.4.2/submodules/