When checking the value of CMAKE_INTERPROCEDURAL_OPTIMIZATION_<CONFIG>,
then <CONFIG> must be the upper case configuration name.
Also, we must not unset __qt_ltcg_detected, because the feature
condition evaluation happens way after the feature definition, and
__qt_ltcg_detected was not available anymore there.
This commit amends 96bdcdacbc.
Fixes: QTBUG-87874
Change-Id: I2b555bd1d7fbc8973e1d9493045c27d61b0d2c1b
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
e559da3c6a