Qt: enhance cross-compilation support for desktop platforms

[Doekin]

This PR improves cross-compilation workflows by introducing a new qt-tools package, which provides the essential host tools required for building Qt applications in cross-compilation environments.

Example Usage

For a simple Qt Widgets application created using the command:

xmake create -l c++ -t qt.widgetapp

Update the xmake.lua file as follows:

add_rules("mode.debug", "mode.release")
+ add_requires("qt6widgets")

target("test")
    add_rules("qt.widgetapp")
+   add_packages("qt6widgets")

    add_headerfiles("src/*.h")
    add_files("src/*.cpp")
    add_files("src/mainwindow.ui")
    -- Add files with Q_OBJECT meta (only for qt.moc)
    add_files("src/mainwindow.h")

To build the project for MinGW on Linux, run:

xmake f -c -vD -p mingw

This command will automatically install all required packages, including qt6base, qt6core, qt6gui, and qt-tools.

Alternatively, we can specify a host Qt SDK using the --qt_host option:

xmake f -c -vD -p mingw --qt_host=<host Qt>

• If the host Qt SDK version matches the target Qt SDK version, the host tools will be used directly.
• If the versions differ, the qt-tools package will be installed to ensure compatibility.

[SirLynix]

I'm not sure what qttools package brings here, cross-compilation is supported by qtcore by having it install the tools when cross-compiling, why not just extend this logic?

[Doekin]

The main reasons for using qt-tools are:

1. Reusing Host Tools: This avoids downloading the full Qt SDK every time we cross-compile for different platforms.
2. Leveraging Existing SDKs: If a suitable SDK is already installed, it can be detected and used directly, skipping redundant installations.

I hope this clarifies your concerns.

[SirLynix]

Thanks for the clarification, it seems like a nice change indeed.

[Doekin]

I just noticed a potential issue in both the dev branch and my PR. If qt6base is installed, qt6core, qt6gui, and qt6widgets are considered installed as well, though their dependencies may still be missing. I'll mark my PR as a draft to investigate and address this. Suggestions are welcome—thank you!

[Doekin]

If qt6base is installed, qt6core, qt6gui, and qt6widgets are considered installed as well, though their dependencies may still be missing.

Resolved by adding the following check to on_fetch in qt6lib:

for _, dep in ipairs(package:plaindeps()) do
    if not dep:fetch() and dep:parents(package:name()) then
        return
    end
end