Good point. The C++ wrapper already defaults to UFSECP_LBTC_AUTO, and I updated the libbitcoin docs/examples to use that form:
static thread_local ufsecp::lbtc::Controller context;
So libbitcoin does not need to spell the backend enum at normal call sites. The raw C ABI remains explicit. @pmienk can review the build profile here:
https://github.com/shrec/UltrafastSecp256k1/blob/dev/docs/LIBBITCOIN_INTEGRATION.md#build-profile