fix xdr detection and use with recent glibc

fix xdr detection and use with recent glibc

glibc 2.13.90 has obsoleted its rpc implementation in favour of
the one provided by the TI-RPC library:

> * The RPC implementation in libc is obsoleted.  Old programs keep working
>   but new programs cannot be linked with the routines in libc anymore.
>   Programs in need of RPC functionality must be linked against TI-RPC.
>   The TI-RPC implemtation is IPv6 enabled and there are other benefits.
>
>   Visible changes of this change include (obviously) the inability to
>   link
>   programs using RPC functions without referencing the TI-RPC library,
>   the
>   removal of the RPC headers from the glibc headers, and the lack of
>   symbols defined in <rpc/netdb.h> when <netdb.h> is installed.
>   Implemented by Ulrich Drepper.
(from glibc NEWS)

Thus with recent glibc, we need to try linking with -ltirpc when looking
for the XDR functions. The daemon also needs to use XDR_CFLAGS to be able
to find the XDR headers stored in /usr/include/tirpc.

When using TI-RPC, there are some warnings about redundant declarations, but
the fix probably belongs in other modules:

/usr/include/tirpc/rpc/rpcent.h:68:13: warning: redundant redeclaration of
'setrpcent' [-Wredundant-decls]
/usr/include/rpc/netdb.h:53:13: note: previous declaration of 'setrpcent'
was here

/usr/include/tirpc/rpc/rpcent.h:69:13: warning: redundant redeclaration of
'endrpcent' [-Wredundant-decls]
/usr/include/rpc/netdb.h:54:13: note: previous declaration of 'endrpcent'
was here

/usr/include/tirpc/rpc/rpc.h:84:12: warning: redundant redeclaration of
'bindresvport' [-Wredundant-decls]
/usr/include/netinet/in.h:440:12: note: previous declaration of
'bindresvport' was here