I was working on some install manifest foo on a plane and found a bug in dependencies. Essentially, the install manifest wasn't being recorded as a dependency. So if the set of defines changed (stored in the install manifest), we didn't redo preprocessing. I'm sure this bug is responsible for at least one clobber in the past few months. I'm too lazy to dig through all the open bugs. Patch forthcoming.

The install manifest processor starts with an empty InstallManifest and uses |= to "concatenate" instances. It became pretty obvious when developing some patches that add more preprocessed files to install manifests that the source install manifest dependency was getting lost during the |= operation. This patch fixes it. The solution is not ideal performance wise. But slightly worse performance (only after config.status, however) is better than clobbers. A test has been added to ensure this doesn't regress.