Any file which includes scmrev.h must be rebuilt when scmrev.h
is regenerated. By not including scmrev.h from any file other
than Version.cpp, incremental builds become a little faster.
Ideally Common.h wouldn't be a header in the Common library, and instead be renamed to something else, like PlatformCompatibility.h or something, but even then, there's still some things in the header that don't really fall under that label
This moves the version strings out to their own version header that doesn't dump a bunch of other unrelated things into scope, like what Common.h was doing.
This also places them into the Common namespace, as opposed to letting them sit in the global namespace.
