qmake make install with multiple jobs leads to failure

The makepkg in Manjaro Linux adds by default

-j $JOBS

(which should be probably changed in arch/manjaro)

I have seen randomly failing builds of many packages depending of some race condition. I believe, the qmake generates install target of Makefile incorrectly.

steps to reproduce: Take some non-trivial qmake based project and run make install many times and watch for failures:

set -e; for i in $(seq 1 100); do make -j 20 install; echo $?; done

Debug output in my project is here:
https://github.com/nemomobile-ux/glacier-dialer/pull/7