Summary:
We didn't do a great job of recognizing that we'd need to
build a project when one of its dependencies had changed: we relied
chiefly on the dependency hash for this and could fail to handle
changes in individual source files.

This commit helps to improve this situation by checking to see if
any installed files in the dependencies of a manifest are newer than
the most recent built time of a given manifest.  If so, we'll trigger
a build.  We try to be reasonably smart about deciding when to trigger
a cmake reconfigure if it looks like cmake files in the deps have
been changed.

Reviewed By: xavierd

Differential Revision: D21364132

fbshipit-source-id: 7534496e10d1f532aa9cf865900ace84a8785327