This won't compile anymore and needs to be updated.
ngx_http_auth_yubikey_module.c:50:18: error: ‘MD5_DIGEST_LENGTH’ undeclared here (not in a function); did you mean ‘SHA_DIGEST_LENGTH’?
u_char md5[MD5_DIGEST_LENGTH];
^~~~~~~~~~~~~~~~~
SHA_DIGEST_LENGTH
make[1]: *** [objs/Makefile:1849: objs/addon/ngx_http_auth_yubikey_module/ngx_http_auth_yubikey_module.o] Error 1
This won't compile anymore and needs to be updated.