Looks like it isn't just nginx (or even more likely IS an openssl issue) because I just encountered it again with pjproject:
*****
../src/pj/ssl_sock_ossl.c:1001:5: warning: implicit declaration of function ‘M_ASN1_STRING_length’ [-Wimplicit-function-declaration]
len = M_ASN1_STRING_length(X509_get_serialNumber(x));
^
../src/pj/ssl_sock_ossl.c: In function ‘pj_ssl_sock_get_info’:
../src/pj/ssl_sock_ossl.c:2285:24: error: dereferencing pointer to incomplete type
info->cipher = (cipher->id & 0x00FFFFFF);
^
*****