MacOS: Handle changes to xdrproc_t definition

MacOS: Handle changes to xdrproc_t definition

With Mac OS X 10.9, xdrproc_t is no longer defined as:

typedef bool_t (*xdrproc_t)(XDR *, ...);

but instead as:

typdef bool_t (*xdrproc_t)(XDR *, void *, unsigned int);

For reference, Linux systems typically define it as:

typedef bool_t (*xdrproc_t)(XDR *, void *, ...);

The rationale explained in the header is that using a vararg is
incorrect and has a potential to change the ABI slightly do to compiler
optimizations taken and the undefined behavior. They decided
to specify the exact number of parameters and for compatibility with old
code decided to make the signature require 3 arguments. The third
argument is ignored for cases that its not used and its recommended to
supply a 0.