Use cmake to generate the version.h from version.h.in which is a cleaned
up version of the old version.h with the git short sha into the build
directory, and include the version.h from there.

Continue to use the GetGitRevisionDescription plugin to make the cmake
configuration state depend on the current sha of HEAD, but throw away
the results (for the time being.)

This makes rebuilds after git changes such as a commit only recompile a
couple of files instead of the whole tree.