support gcc-16 with no PCH and no unity build

[stefantalpalaru]

Resolves: https://gcc.gnu.org/gcc-16/porting_to.html#missing-header-includes

Compiling Audacity-4.0.0 with gcc-16 fails due to missing headers.
Disabling unity builds (-DMUSE_COMPILE_USE_UNITY=OFF) and PCH (-DMUSE_COMPILE_USE_PCH=OFF) brings further failures, due to incomplete Qt header paths.

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Build configuration

audacity: audacity/audacity/master
audacity platforms: linux_x64
musescore: musescore/MuseScore/main
musescore platforms: linux_x64

[cbjeukendrup]

It is quite unexpected that QtQuick/ is necessary here; especially given that it is apparently not necessary at for example <QQmlListProperty>. Do you have any explanation for that?

[stefantalpalaru]

Won't compile otherwise, on Gentoo ~amd64, with Qt 6.11.2.

Here's the output of `rg -i 'include.*qquickitem>' /usr/include/qt6/`:

/usr/include/qt6/QtQuickParticles/6.11.2/QtQuickParticles/private/qquickparticlesystem_p.h
18:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuickTestUtils/6.11.2/QtQuickTestUtils/private/viewtestutils_p.h
20:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuickParticles/6.11.2/QtQuickParticles/private/qquickparticleemitter_p.h
18:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuick/qquicktextdocument.h
9:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuickLayouts/6.11.2/QtQuickLayouts/private/qquicklayout_p.h
20:#include <QQuickItem>

/usr/include/qt6/QtQuick3D/6.11.2/QtQuick3D/private/qquick3dtexture_p.h
24:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtChartsQml/6.11.2/QtChartsQml/private/declarativepolarchart_p.h
18:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtChartsQml/6.11.2/QtChartsQml/private/declarativepieseries_p.h
21:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtChartsQml/6.11.2/QtChartsQml/private/declarativeboxplotseries_p.h
22:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtChartsQml/6.11.2/QtChartsQml/private/declarativechart_p.h
22:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtChartsQml/6.11.2/QtChartsQml/private/declarativecandlestickseries_p.h
21:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtChartsQml/6.11.2/QtChartsQml/private/declarativebarseries_p.h
27:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuick/qquickframebufferobject.h
8:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuick/qquickrhiitem.h
8:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuick3D/6.11.2/QtQuick3D/private/qquick3dviewport_p.h
20:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuick/6.11.2/QtQuick/private/qquickanimatedsprite_p.h
23:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuick/6.11.2/QtQuick/private/qaccessiblequickitem_p.h
19:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuick/6.11.2/QtQuick/private/qquickspritesequence_p.h
23:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtPdfQuick/6.11.2/QtPdfQuick/private/qquickpdfselection_p.h
25:#include <QtQuick/QQuickItem>

/usr/include/qt6/QtQuickVectorImage/6.11.2/QtQuickVectorImage/private/qquickvectorimage_p.h
18:#include <QQuickItem>

/usr/include/qt6/QtQuickControls2Impl/6.11.2/QtQuickControls2Impl/private/qquicktumblerview_p.h
19:#include <QQuickItem>

/usr/include/qt6/QtQuickVectorImageGenerator/6.11.2/QtQuickVectorImageGenerator/private/qquicknodeinfo_p.h
22:#include <QQuickItem>

/usr/include/qt6/QtQuickEffects/6.11.2/QtQuickEffects/private/qgfxsourceproxy_p.h
19:#include <QtQuick/QQuickItem>