it makes no sense to let every spec do that separately, as it's fixed
by the generator+shell.
putting it into a file which is loaded regardless of the spec also
allows us to remove the hardcoded fallbacks from qmake.
if somebody overrode the values in their spec for some weird reasons,
they'll need to override spec_post.prf.
shell-{unix,win32}.conf are now dummies and print warnings.
Task-number: QTBUG-37269
Change-Id: I66c24fb4072ce4d63fdbfc57618daa2a48fa1d80
Reviewed-by: Jochen Seemann <seemann.jochen@gmail.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
95d385466d