BUILD: ssl: fix build error introduced in commit 7969a3 with OpenSSL < 1.0.0

BUILD: ssl: fix build error introduced in commit 7969a3 with OpenSSL < 1.0.0

The function 'EVP_PKEY_get_default_digest_nid()' was introduced in OpenSSL
1.0.0. So for older version of OpenSSL, compiled with the SNI support, the
HAProxy compilation fails with the following error:

src/ssl_sock.c: In function 'ssl_sock_do_create_cert':
src/ssl_sock.c:1096:7: warning: implicit declaration of function 'EVP_PKEY_get_default_digest_nid'
   if (EVP_PKEY_get_default_digest_nid(capkey, &nid) <= 0)
[...]
src/ssl_sock.c:1096: undefined reference to `EVP_PKEY_get_default_digest_nid'
collect2: error: ld returned 1 exit status
Makefile:760: recipe for target 'haproxy' failed
make: *** [haproxy] Error 1

So we must add a #ifdef to check the OpenSSL version (>= 1.0.0) to use this
function. It is used to get default signature digest associated to the private
key used to sign generated X509 certificates. It is called when the private key
differs than EVP_PKEY_RSA, EVP_PKEY_DSA and EVP_PKEY_EC. It should be enough for
most of cases.