unbreak build by moving away from deprecated exec_program
unbreak build by moving away from deprecated exec_program

Summary:
since 1137ed5f07d137296874f7dd1ee629f903507161 which bumped cmake and
started using TARGET_FILE for the qmake calls qca failed to build for me.
I fail to understand why exactly that is but I suspect it has to do with
the fact that exec_program is deprecated and may not be working
(correctly?) with generator expressions.

moving to the not deprecated execute_process fixes clean builds on my
system and is at the very least a move away from deprecated tech with
no downsides

Test Plan: rm -rf * && cmake -DCMAKE_INSTALL_PREFIX=/usr ..

Reviewers: asturmlechner, dfaure, vonreth

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D20228

KDE/qca,KDE/qca,KDE/qca,KDE/qca