Fix use in cross compilation

Summary:
This changes make kmag cross-compile with libqaccessibilityclient. Key difference is to make paths in generated QAccessibilityClient-release.cmake not fixed by using KDE_INSTALL_TARGETS_DEFAULT_ARGS. Lines reordering was needed because cmake skips recognizing compiler otherwise.
Patch is shaped after libkomparediff2's cmake file that works well.

Test Plan: Patch was used to cross build in Void Linux, kmag works.

Reviewers: jriddell, kossebau

Reviewed By: kossebau

Differential Revision: https://phabricator.kde.org/D21233