with the system-reserved (and system-defined) '_REENTRANT' 
CPP identifier.
Especially, it must not be "undefine"d, because the Tru64 compiler
needs it if threads are enabled.


include/my_pthread.h:
  On Tru64, '_REENTRANT' is set by the compiler when the "-pthread" 
  option is given, and it must be set when the system's 'pthread.h' is 
  included (or else a '#error' is activated).
  So it must not be undef'ed, and it should not be def'ed either, as it
  is a system-reserved CPP identifier with which we should not mess
  anyway - it is risky to use it!
  Build on all platforms was checked privately, change caused no errors!