Clean up git version generation.

Clean up git version generation.

- Rename USE_VERSION_H to GMX_GIT_VERSION_INFO. This was particularly
  confusing now that there was a separate version.h file for API version
  numbers.
- Move the version.h/version.c files to src/gromacs/utility/ and rename
  them to gitversion.*, again to avoid confusion with the API version.h
  header.
- Remove unnecessary suffix stripping from gmxGenerateVersionInfo.cmake:
  when this functionality was added, the version string was of the form
  X.Y-dev-YYYYMMDD, and the script was stripping the date off. Now that
  the date is gone, it is clearer to just append the generated version
  with no extra logic.
- Change the logic for handling GMX_GIT_VERSION_INFO in CMake such that
  user-provided cache values are never changed under the hood.

Change-Id: I372a4e2fdf11b1799dd22997bc6adb4faf3db93d