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).
el_runtime.c uses OpenSSL (EVP_*, RAND_bytes) for AEAD encrypt/decrypt.
elb was only linking -lcurl -lpthread -lm, missing the SSL libs.
Matches the explicit flags used in ci-dev.yaml and ci-stage.yaml.
gcc rejects -fbracket-depth=1024 with 'unrecognized command-line option'.
Use shell subshell to probe cc --version and only pass the flag when
the compiler is clang.