LibreSSL has long made EVP_ENCODE_CTX opaque.

Index: xsec/enc/OpenSSL/OpenSSLCryptoBase64.cpp
--- xsec/enc/OpenSSL/OpenSSLCryptoBase64.cpp.orig
+++ xsec/enc/OpenSSL/OpenSSLCryptoBase64.cpp
@@ -48,7 +48,7 @@ XERCES_CPP_NAMESPACE_USE
 //           Construction/Destruction
 // --------------------------------------------------------------------------------
 
-#if (OPENSSL_VERSION_NUMBER < 0x10100000L) || defined(LIBRESSL_VERSION_NUMBER)
+#if (OPENSSL_VERSION_NUMBER < 0x10100000L) || defined(LIBRESSL_VERSION_NUMBER_TOO_OLD)
 OpenSSLCryptoBase64::OpenSSLCryptoBase64() : mp_ectx(&m_ectx_store), mp_dctx(&m_dctx_store) { }
 OpenSSLCryptoBase64::~OpenSSLCryptoBase64() { }
 #else