Doesn't build with OpenSSL 1.1

[zofrex]

I got the project building fine on my machine with OpenSSL 1.0.2t, but get a compilation error with 1.1.1g:

/Users/zofrex/SoftU2F/SelfSignedCertificate/SelfSignedCertificate.m:113:14: error: variable has incomplete type 'EVP_MD_CTX' (aka 'struct evp_md_ctx_st')
  EVP_MD_CTX ctx;
             ^
In file included from /Users/zofrex/SoftU2F/SelfSignedCertificate/SelfSignedCertificate.m:8:
In file included from /Users/zofrex/SoftU2F/SelfSignedCertificate/private.h:13:
In file included from /usr/local/opt/openssl/include/openssl/asn1.h:16:
In file included from /usr/local/opt/openssl/include/openssl/bio.h:20:
In file included from /usr/local/opt/openssl/include/openssl/crypto.h:25:
/usr/local/opt/openssl/include/openssl/ossl_typ.h:92:16: note: forward declaration of 'struct evp_md_ctx_st'
typedef struct evp_md_ctx_st EVP_MD_CTX;
               ^
1 error generated.

From a quick Google it seems like this is due to changes between 1.0->1.1, and there'll probably be other things to fix too.

Given that OpenSSL 1.0 is no longer supported, I think support for 1.1 should be added.

I'm happy to have a go at that but thought I would ask about it before starting / making a PR!

If upgrading to 1.1 is desirable does backward compatibility need to be kept to work with 1.0 as well, or can 1.0 support be completely dropped?