for portability reasons rpc.h should be used instead of xdr.h, change

the test accordingly

[[Imported from SVN: r1236]]

m4/xdr.m4

 # $Id$
-# searches for XDR Headers 
+# searches for XDR Headers which are implicitly included by rpc.h
+# some systems don't like it when xdr.h is directly included
 
 AC_DEFUN([DUNE_PATH_XDR],[
   AC_REQUIRE([AC_PROG_CC])
-  AC_CHECK_HEADERS(rpc/xdr.h)
+  AC_CHECK_HEADERS(rpc/rpc.h)
 ])