Add AUIB_AUI_OVERRIDE_VERSION in addition to AUIB_AUI_AS

When using AUIB_AUI_AS in order to develop AUI alongside with AUI based (client) application, I often make commits to AUI. I also have to update commit sha1 (version) in auib_import(AUI) in the client application, so other developers would receive AUI update. However, the local AUI repository location on my machine is also updated, and I have to do `git remote origin set-url` in AUI's repository in order to make commit every time. The idea is to **override** the AUI version only on my machine with CMake argument, so the repository wouldn't updated when I update AUI version in the client application, and I wouldn't have to setup remote origin url in order to make a commit to AUI.

No files or tests are named. Start by tracing AUIB_AUI_AS through auib_import(AUI) and the CMake argument handling; done means a local CMake override can select an AUI version without updating the repository reference or requiring a changed remote origin URL.

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