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