Release 0.12.1

• Fix compilation in buildroot (#207)
• Fixes regarding OpenSSL (#208)
• Fix some build steps when builddir != srcdir (#208)
• Allow the user to disable build of examples (#209)
• CI builds against OpenSSL 3 (#206)
• Change the call signature of the following API:
  • xmpp_conn_set_client_cert() - the PKCS#12 file has now to be passed via the cert
    parameter. Originally it was via key. Currently both styles are supported,
    but in a future release only passing via cert will be accepted.