[PATCH 2 of 2] SSL: let it build against BoringSSL

[Piotr Sikora]

Hey Maxim,

> The problem is lots of added lines required to avoid calling it.

Alternatively, would you be OK with a change that adds #ifndefs around
SSL_CTX_set_tmp_rsa_callback() in ngx_http_ssl_module and
ngx_mail_ssl_module, but keeps ngx_ssl_rsa512_key_callback without
guards? It's exported, so compilers won't complain about it being
unused.

Best regards,
Piotr Sikora