Add `c_source "path"` in manifest.el build block — lets packages link
extra C files (platform stubs, native glue) without touching elb source.
On macOS, homebrew OpenSSL isn't on the default linker path. Detect it
via `brew --prefix` and inject -L/-I flags; no-op on Linux.
Rebuild elb binary; remove elc-new binary (elc is now canonical).