qt5 cross compilation

[ghost]

The PRs below will allow to cross-compile all of QT-5 except qtwebengine. I'm working on that. I'll close this issue when it happens. In the meantime, any clues on why qtwebengine fails like this are welcome:

nix build -f . -L pkgsCross.aarch64-multiplatform.qt5.qtwebengine
...
qtwebengine-aarch64-unknown-linux-gnu> Project ERROR: Unknown module(s) in QT: qml-private quick-private
qtwebengine-aarch64-unknown-linux-gnu> make[2]: *** [Makefile:65: sub-core_generator-pro-make_first] Error 3
qtwebengine-aarch64-unknown-linux-gnu> make[2]: Leaving directory '/build/source/src/core'
qtwebengine-aarch64-unknown-linux-gnu> make[1]: *** [Makefile:79: sub-core-make_first] Error 2
qtwebengine-aarch64-unknown-linux-gnu> make[1]: Leaving directory '/build/source/src'
qtwebengine-aarch64-unknown-linux-gnu> make: *** [Makefile:49: sub-src-make_first] Error 2

The interwebs say it's because qmake can't find qtdeclarative.

• qt-5: add buildPackages.stdenv.cc to nativeBuildInputs #220310
• qt-5/hooks/fix-qt-builtin-paths.sh: do not hardcode output paths #220311
• qt-5/modules/qtbase.nix: set flags for cross compilation #220314
• qt5/qtbase.nix: add mysqlSupport #220312
• qt-5/modules/qtbase.nix: add configureFlags for cross #220313
• qt-5/modules/qtbase.nix: set flags for cross compilation #220314
• qt-5/modules/qtbase.nix: omit --host and --build configureFlags #220315
• qt5/qtwebchannel.nix: omit $bin output if cross #220316
• qt5.qtwebengine: point build at proper pkg-config for cross #220319
• qt5: inherit from __splicedPackages to fix cross #220374
• v4l-utils: move wrapQtAppsHook to buildInputs #220431