Reacting on xorriso ifailure in pipe test of releng/auto_isocontent

releng/README

@@ -139,8 +139,12 @@ All general options are accepted.
   Exercises inclusion of xorriso/xorriso.h and libisoburn/libisoburn.h
   in C++ programs and linking of the libraries. It might be necessary
   to set compiler options by shell variable CFLAGS before running the test.
+  It might be necessary to hand over the install directory of libburn and
+  libisofs in shell variable LD_LIBRARY_PATH.
   E.g. if on FreeBSD the include headers libisofs.h , libburn.h are not found:
     export CFLAGS="-I/usr/local/include"
+  E.g. on GNU/Hurd, where libburn and libisofs are not found by the linker:
+    export LD_LIBRARY_PATH="/usr/local/lib"
 
 ./auto_isocontent -x ../xorriso/xorriso
   Tests whether xorriso is able to record and restore two familes of