Configuring Qt Creator against self-built Qt fails (could not find QtCompilerFlags)

In an installed Qt 6.3, there is a file /lib/cmake/Qt6/QtCompilerFlags.cmake which Qt Creator finds.
In a self-built Qt 6.3, the file is here /cmake/QtCompilerFlags.cmake which Qt Creator does not find:

...
– Found XKB: /usr/lib/x86_64-linux-gnu/libxkbcommon.so (Required is at least version "0.5.0")
– Found WrapVulkanHeaders: /usr/include
CMake Error at CMakeLists.txt:78 (include):
include could not find requested file:

QtCompilerFlags
...