update version patchlevel from svn revision to date

With the move to git we no longer have a short monotonically increasing integer to use for the version patchlevel for unofficial builds (for identifying builds as well as allowing dependencies to be expressed between releases).

The proposal is to switch to using the date of the last commit. With our rebase workflow, the committer date (not author date) should fit our purposes.

For builds outside of a git repo, e.g., from a source tarball, we should support getting the patchlevel from a file. We should switch package.cmake to use this for official builds as well. If that file is not present and we're in a git repo, we'll then use the date of the last commit.

Getting something in place is high priority as the Windows build is broken due to this.