Before this commit, the following will trigger a complete rebuild:

- create another git name (git checkout -b test)
- run cmake again manually (cmake ..)
- run ninja again

The top-level CMakeLists.txt uses add_definitions() to add git
information to all targets; if some git information changes, this will
mark all command lines as dirty, triggering the build.

There is an easy solution: instead of passing this information on the
command line, pass it in a header. This commit uses configure_file() to
generate a file oai_version.h in the build directory with the same
information. Various sources files, e.g., main executables, use this to
output the same information as before. It is then also not necessary to
use header guards, as cmake takes care to create this file.

If cmake is re-run, it will update the header, and trigger only files
including this header will be rebuilt (which can be shown with the above
stpes). The information passed is the same as before, and updates the
information when the old method would have updated it.