Copying from the 4.10 Release Schedule page
Thursday, November 1, 2012:
From this moment on it is not allowed to add new dependencies or bump dependencies versions. It is possible to get an exception for this. Post the patch to reviewboard and add the release-team as reviewer. We will check if the dependency is needed and is available on all platforms.
In other words: If you have a feature that requires a new dependency or a version of a dependency that is higher than currently checked for in the build system, you need to have committed this change before this date.