update version patchlevel from svn revision to date

[derekbruening]

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.

[derekbruening]

Commit 0c81acf added the day of the last commit to solve the immediate build problem on Windows.

Leaving open for the packaging and source tarball support.

[derekbruening]

GitHub has a feature "Download Zip" that lets people easily obtain the source files without any version control. This will be a pain to support, as A) it does not include git submodules and B) adding patchlevel support would require us to update a file on every commit. I propose that we simply do not support building from such zip files. I do not see any way to disable the feature from the UI.