Fix #75282: xmlrpc_encode_request() crashes

Fix #75282: xmlrpc_encode_request() crashes

Since we allow ext/xmlrpc to be built against a system libxmlrpc(-epi),
we must not `efree` memory which has been allocated via `malloc`.  To
distinguish bundled and system libxmlrpc(-epi) we introduce the macro
`HAVE_XMLRPC_BUNDLED` (analogous to how it is done by ext/gd).  We
deliberately keep the ugly `#ifdef`s, instead of tucking them away in
an `XMLRPC_FREE()` macro, to not forget that it is a bad idea to fork
and bundle a library, but to also allow building against an unpatched
system lib.