Compiler breakage when strange CPU meets new OpenSSL

If you manage to get yourself a system where we don't define USE_EVP_AES_CTR (eg, ARM + Sid), and we have a very recent (1.1?) OpenSSL, you get these errors:

src/common/aes.c:147:20: error: field ‘evp’ has incomplete type
     EVP_CIPHER_CTX evp;
                    ^~~
src/common/aes.c: In function ‘evaluate_ctr_for_aes’:
src/common/aes.c:243:5: error: implicit declaration of function ‘AES_ctr128_encrypt’ [-Werror=implicit-function-declaration]
     AES_ctr128_encrypt(&zero[i], &output[i], 1, &key, ivec, ivec_tmp, &pos);
     ^~~~~~~~~~~~~~~~~~
src/common/aes.c:243:5: error: nested extern declaration of ‘AES_ctr128_encrypt’ [-Werror=nested-externs]
  CC       src/common/crypto_s2k.o
cc1: all warnings being treated as errors