lib/replace/wscript:Tru64 build - Better use version 600 for _XOPEN_SOURCE

lib/replace/wscript:Tru64 build - Better use version 600 for _XOPEN_SOURCE

This means we request the "Single UNIX Specification, Version 3" with C99
compatibility as the Python 2.5 release on the system. This prevents
redefinitions with different values.

> [ 451/1918] Compiling scripting/python/pyglue.c
> cc: Warning: /usr/local/include/python2.5/pyconfig.h, line 951: The redefinition of the macro "_XOPEN_SOURCE" conflicts with a current definition because the replacement lists differ.  The redefinition is now in effect. (macroredef)
> #define _XOPEN_SOURCE 600
> ----------------------^
> cc: Warning: default/source4/include/config.h, line 54: The redefinition of the macro "_XOPEN_SOURCE" conflicts with a current definition because the replacement lists differ.  The redefinition is now in effect. (macroredef)
> #define _XOPEN_SOURCE 500
> ----------------------^