fix relative rpaths on linux

$ORIGIN (or $LIB) needs to be escaped to survive the trip through
make and the shell.

this shouldn't break anything, as there was simply no way to get it
right so far.

Change-Id: I86337c5994d10dae2e80dd2f858f74874b14bca7
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>