Before 7142b04337d6, it was possible to build the OpenSSL library
along with nginx, and link nginx statically with this library
(--with-openssl=DIR --with-ld-opt=-static --with-http_ssl_module).
This was broken on Linux by not adding -ldl after -lcrypto.
The fix also makes it possible to link nginx statically on Linux
with the system OpenSSL library, which never worked before.