qt6: add initial version for building and packaging base of Qt6

[mirko]

Qt6 is a largely complex beast and given the experience from previous ports of Qt4 and Qt5, it will require quite some more work to accommodate all needs and configurations.

Qt6 kind of ditched support for qmake in favour of cmake, at least when it comes to building Qt6 and its official modules itself.

The qmake eco-system is and still needs to be supported though, as 3rd party projects are still allowed to use qmake - for the time being at least. qmake support for external projects untested for the time being, though.

[openwrt]

Formality Check: Failed

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🛑 CRITICAL ERRORS

Commit 2a3b2e7 - qt6: add initial version for building and packaging base of Qt6:

• New OpenWrt package is missing the mandatory parameter: 'PKG_MAINTAINER'
• New OpenWrt package is missing the mandatory parameter: 'PKG_LICENSE'
• New OpenWrt package is missing the mandatory parameter: 'PKG_LICENSE_FILES'
• Makefile line 'DEPENDS+=+zlib +libzstd +libstdcpp +libpcre2-16 +libdouble-conversion $(ICONV_DEPENDS) +BUILD_qt6base-gui_ICU:icu #FIXME: do not include ICONV_DEPENDS if ICU is selected (though, that's only an issue when using GNU iconv)' inside 'Package/qt6base-core' must be indented with exactly 2 spaces
• Makefile line 'TITLE+=bearer (plugin)' inside 'Package/qt6base-plugin-bearer-generic' must be indented with exactly 2 spaces

Tip

Do not close this pull request to make corrections. Instead, modify your existing commits (e.g. git commit --amend) and update the branch using git push --force-with-lease --force-if-includes. The checks will re-run automatically.

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_{Something broken? Consider reporting an issue.
Running version 516b28e deployed on 2026-07-26 09:31:58 CEST}