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Fix warning about missing or non-existing QTDIR #569

Job	Run time
ubuntu (22.04)	1m 25s
ubuntu (24.04)	1m 22s
macOS (latest, 3.9, 5.9.*, release)	6m 57s
macOS (latest, 3.9, 5.9.*, debug)	6m 48s
macOS (latest, 3.11, 5.12.*, debug)	6m 23s
oldschool (rockylinux, 9, debug)	2m 21s
windows (win32_mingw53, 3.6, x86, 5.11.*)	3m 16s
windows (win64_msvc2017_64, 3.6, x64, 5.11.*)	2m 19s
oldschool (rockylinux, 9, release)	2m 19s
windows (win64_mingw73, 3.10, 5.12.*, x64)	2m 28s
windows (win64_msvc2019_64, 3.13, x64, 5.15.*, PythonQtCore PythonQtGui PythonQtMultimedia)	1m 40s
	37m 18s

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