Add config_summary to Spiner.

[BrendanKKrueger]

PR Summary

As with Ports-of-Call:

• The config_summary stuff is something we put together for Singe because we've found it useful in other projects. If you're not interested in adding this to Spiner, feel free to close this MR.
• This is just an attempt to add config_summary and may not be set up the way you like. Feel free to tweak it or ask me to adjust things.

I'm not positive, but it seems like spiner_content_declare may not be setting up the <packagename>_VERSION variables, which causes the output from config_summary to look weird. This may suggest I need to fix config_summary, someone needs to fix spiner_content_declare, or Spiner just shouldn't mix the two together.

Example screenshot of config_summary output:

PR Checklist

• Code is formatted. (You can use the format_spiner make target.)
• Adds a test for any bugs fixed. Adds tests for new features.
• If preparing for a new release, update the version in cmake.

[BrendanKKrueger]

I ran into this issue elsewhere (for example lanl/ports-of-call#65), and the issue is probably that we need to export the version flag to the parent scope. But I may have to play with things a bit to get spiner_content_declare to handle it all correctly, because it's not as simple as searching for find_package and adding the appropriate set command. Making a note for when someone (possibly future-me) gets around to addressing this.