Merge pull request #1792 from CEED/jed/runtime-config

Add run-time interface for build-time configuration

Add run-time interface for build-time config (fix #1732)

This adds CeedGetGitVersion() to access output of `git describe --dirty`
and CeedGetBuildConfiguration() to get variables like compilers and

Add run-time interface for build-time config (fix #1732)

This adds CeedGetGitVersion() to access output of `git describe --dirty`
and CeedGetBuildConfiguration() to get variables like compilers and
flags.

For builds without a Git repo, we return "unknown" or the user can set
the string via `make GIT_DESCRIBE=deadbeaf`.

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