in bintars the server is linked with wolfssl, while the connector
is linked with gnutls. Thus client_ed25519.so gets gnutls
dependency, unresolved symbols and it cannot be loaded into the
server and gnutls symbols aren't present there.
linking the plugin statically with gnutls fixes that and the test passes.
but when such a plugin is loaded into the client, the client gets
two copies of gnutls - they conflict and ssl doesn't work at all.
let's detect this and disable the test for now.
realloc can't be done for mysql->passwd. There's also no pretty way to
pass the ext-salt into hash_password, so let's use thread_local instead.
For GCC < 4.9 (SLES 12, CentOS 7), which don't support _Thread_local,
pk will be recalculated instead
