SSL: avoid using OpenSSL config in build directory (ticket #2404). With this change, the NGX_OPENSSL_NO_CONFIG macro is defined when nginx is asked to build OpenSSL itself. And with this macro automatic loading of OpenSSL configuration (from the build directory) is prevented unless the OPENSSL_CONF environment variable is explicitly set. Note that not loading configuration is broken in OpenSSL 1.1.1 and 1.1.1a (fixed in OpenSSL 1.1.1b, see https://github.com/openssl/openssl/issues/7350). If nginx is used to compile these OpenSSL versions, configuring nginx with NGX_OPENSSL_NO_CONFIG explicitly set to 0 might be used as a workaround.