/usr/lib64/sssd/libsss_proxy.so has references to dlsym, dlopen, dlerror, so it needs to be linked with -ldl. Not doing so risks crashes at run time.
sssd has quite complicated dependencies and cannot be easily compiled with -Wl,-z,defs. I tried to fix as much as possible and this missing fix is part of related PR https://github.com/SSSD/sssd/pull/502/files#diff-c949f93d03f44a4217d7a138f9e2e54aR3906 BTW we newer noticed the problem because sssd_be is linked with ldl and that binary dynamically open libsss_proxy.so which dynamically open other modules for (pam, nsswitch) based on configuration. Thank you very much for bug report.