There's a merged pull request on the void-packages repository

build-style/qmake: add support for Qt6
https://github.com/void-linux/void-packages/pull/42354

Description:
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There's no `qt6-doc` package (compared to `qt5-doc`) or `qt6-examples` subpackage (compared to `qt5-examples`) but those could just be ignored I suppose

~~There's a big problem: cross-compiling any Qt6 `qmake` package quickly results in something like `make: *** No rule to make target '/usr/libexec/moc', needed by '.moc/main.moc'.  Stop.`~~

~~Native builds still work as expected however already. For a testing tree see [my `qt6-qmake-demo` tree](https://github.com/JamiKettunen/void-packages/commits/qt6-qmake-demo) and try e.g. `./xbps-src pkg qt6-qpa-hwcomposer-plugin` vs `./xbps-src pkg -a aarch64 qt6-qpa-hwcomposer-plugin` on a glibc masterdir.~~

~~As per https://forum.qt.io/topic/137300/qtpaths-return-incorrect-path `ln -s /usr/lib/qt6/libexec/moc /usr/libexec/moc` would probably work as a really ugly hack, but is there someone around (perhaps @Johnnynator?) who knows how this could be fixed properly, and maybe even upstream? Here's a few other places where the same error can be seen:~~
- ~~https://community.nxp.com/t5/i-MX-Processors/Imx8mp-cmake-creat-qt6-project-with-an-error-usr-libexec-moc-no/m-p/1567183~~
- ~~https://gobolinux.discourse.group/t/system-index-libexec-is-empty-issue/101~~

#### Testing the changes
- I tested the changes in this PR: **briefly**

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