When qtbase is configured with CMAKE_STAGING_PREFIX set, then Qt modules
built against this qtbase should also get CMAKE_STAGING_PREFIX by
default. Like CMAKE_INSTALL_PREFIX in regular builds, this prefix will
be determined by the location of QtBuildInternalsExtra.cmake to support
building Qt modules against an installer-provided Qt.
CMAKE_INSTALL_PREFIX on the other hand must be exactly the value that
was provided when building qtbase.
If CMAKE_STAGING_PREFIX is specified by the user, honor it.
To opt out of automatically setting CMAKE_STAGING_PREFIX, set
QT_BUILD_INTERNALS_NO_FORCE_SET_STAGING_PREFIX to ON.
Remove the old code that was supposed to set CMAKE_STAGING_PREFIX.
Pick-to: 6.2 6.3
Fixes: QTBUG-99666
Change-Id: I20edef54c102ca9784fcdef0decf0bd83266ae11
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>