Don't look for .qmake.stash outside build and source directories

I just spent a lot of time (~1 day) trying to figure out why my Qt builds failed, and I know this has happened to a lot of other developers as well, only to figure out it was due to a stray .qmake.stash file in the parent directory of my builds.

Not only is this behavior confusing, it's also incredibly hard to understand that this was the reason building Qt failed.

All I got was a message that atomic integers was not supported on my platform, the reason was that detection of libatomic failed, because a .qmake.stash file outside the source and build directoreis contained an outdated path to the c++ header files.

I.e. from a user perspective:

1. Run configure in the wrong directory once (creates .qmake.stash, a hidden file which is hard to see)
2. Months pass without build problems, because the cached files have the correct values
3. Builds suddenly start failing because cached paths are no longer correct after a system upgrade.
4. Spend hours debugging because neither deleting the source nor build folder has any effect.