When a Qt module Foo links against a Qt module Bar then we add the ${QT_BUILD_DIR}/include/QtBar/QtBar header to the list of precompiled header files. However, this only works if Foo and Bar are in the same build system, e.g. in the same repository or in a top-level build. For example, the QtCore header was never precompiled when in a per-repo build of QtSvg. Now, we determine whether Foo and Bar are in the same build system. If they are, we proceed as before. Otherwise, we calculate the QtBar header's location from the following data: - the location of Qt6BarConfig.cmake - the relative path to Bar's include directory The Q*Application tests' project files had to be adjusted to use NO_PCH_SOURCES now, because they use the source file apphelper_plugin.cpp that specifically checks that pre-compiled headers are not used for its compilation. Fixes: QTBUG-134424 Change-Id: I04ede51e4d853e8b813f660f29be8cfeb684c0b2 Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io> Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>