OPENSSL_config() deprecated in OpenSSL 1.1.0.  Additionally,
SSL_library_init(), SSL_load_error_strings() and OpenSSL_add_all_algorithms()
are no longer available if OPENSSL_API_COMPAT is set to 0x10100000L.

The OPENSSL_init_ssl() function is now used instead with appropriate
arguments to trigger the same behaviour.  The configure test changed to
use SSL_CTX_set_options().

Deinitialization now happens automatically in OPENSSL_cleanup() called
via atexit(3), so we no longer call EVP_cleanup() and ENGINE_cleanup()
directly.

The install_sw target first appeared in OpenSSL 0.9.7e and is documented since
OpenSSL 1.0.0 as the way to install the OpenSSL software without documentation.

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.

This change allows to build nginx against git checkout of OpenSSL.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.