The checks whether we have a Qt prefix build were broken for
top-level builds. Non-prefix top-level builds were incorrectly
detected as prefix builds.
For top-level non-prefix builds QT_HOST_DATA/QT_INSTALL_PREFIX becomes
something like "~/my/build/dir/qtbase" but .qmake.cache (and
.qmake.super) is/are created in "~/my/build/dir".
This patch extends the prefix_build check by probing for the existence
of .qmake.super, which only exists for top-level builds. Also, we add
qt_prefix_build_check.prf as central place for determining whether we
have a prefix build to make sure that qt_configure.prf and
qt_build_config.prf use the same logic.
Fixes: QTBUG-76185
Change-Id: I2b76fe26013496aaf2dac96ea711b06a69550a29
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
