Add option to use vendored openssl, closes #4470

Editor assessment

The subject names the new build option precisely: vendored openssl becomes available, with the requesting issue cited. Type feat and scope core are accurate, mood and length are clean, and the purpose (static-linking environments) is implicit enough for the target audience. No body, which is acceptable for a build-config addition. No footers beyond the PR number. A complete, honest message for a small but meaningful portability feature.