Fix variable definition check

Summary:
We'd check if the variable's value is defined, which is wrong and prevented us
from properly initialising using these deprecated variables.
It would also ignore the value we're passing to the actually right variable when
using it.

Test Plan: Built kstars which needs -DCMAKE_ANDROID_API=24

Reviewers: #frameworks, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D14187