When using run_pro2cmake, multiple pro2cmake processes try to access
and override the condition cache.
Make sure that reads / writes of the cache file are protected by
a file lock, and the content is merged rather than overridden.
This requires use of a new pip package, portalocker.
The script will tell the user to install it if it's missing.
Change-Id: I44798c46ff0912981b186bec40e3e918f249fb4d
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
fbace1f4e0