Restore fail-closed crypto library guard

[NateIsern]

Motivation

• Prevent accidental builds against LibreSSL or unsupported modern OpenSSL-style libcrypto by restoring the original fail-closed guard so the build refuses to proceed by default when RAND_egd is missing, avoiding a consensus-sensitive binary divergence risk.

Description

• Replace the warning-only AC_CHECK_LIB([crypto],[RAND_egd]) behavior with an AC_ARG_WITH([libressl]) block that emits an explicit warning only when --with-libressl is supplied and otherwise fails with AC_MSG_ERROR, restoring the previous fail-closed guard in configure.ac.

Testing

• Verified the change with a targeted Python assertion that confirms the new AC_ARG_WITH([libressl]) block is present and the prior "Continuing anyway" warning is absent, ran git diff --check which passed, and attempted autoconf which failed in this container due to missing Automake/pkg-config macro definitions (expected environment limitation).

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Codex Task