Various minor versioning related tweaks

[jlebon]

commit 75342035d5902f299be53d7b541724407463d62c
Date:   Fri Sep 25 14:59:45 2020 -0400

    Makefile-libostree.am: Uncomment BUILDOPT_IS_DEVEL_BUILD conditional

    We shouldn't have to toggle the conditional itself during release
    builds. It should only evaluate to true during devel builds.

commit 68489f647832e3ab9f3856513e459a0cc296ef84
Date:   Fri Sep 25 15:01:09 2020 -0400

    lib: Minor versioning related fixes

    Fix/add the `Since` marker to the new static delta APIs, and update the
    symbol versioning templates/comments.

[cgwalters]

/lgtm

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