Allow compilation without libtirpc-devel (disables XDR and V3D support).
https://github.com/vectorgraphics/asymptote/commit/3e1e10f75af1d549c40320c57763690c08b22c62

Index: fileio.cc
--- fileio.cc.orig
+++ fileio.cc
@@ -425,6 +425,8 @@ void iofile::writeline()
   if(errorstream::interrupt) throw interrupted();
 }
 
+#if HAVE_LIBTIRPC
+
 void igzxfile::open()
 {
   name=locatefile(inpath(name));
@@ -451,5 +453,7 @@ void igzxfile::closeFile()
     processData().ixfile.remove(index);
   }
 }
+
+#endif
 
 } // namespace camp
