[tlbuild] _XOPEN_SOURCE_EXTENDED and Solaris

[Karl Berry]

    ea> I meant the uposixdefs.h.patch
    
[from AS, http://tug.org/pipermail/tlbuild/2013q1/002493.html]

    #if !defined(_XOPEN_SOURCE_EXTENDED)
    +#ifndef __sun
     #   define _XOPEN_SOURCE_EXTENDED 1
     #endif
    +#endif

Ok, I installed that change.  Seems __sun is not defined on sparc-linux,
so that's good.  (Not that we're compiling on sparc-linux any more.)

BTW, just above this #if, the ICU sources say:
  * Poorly upgraded Solaris machines can't have this defined.
  * Cleanly installed Solaris can use this #define.
Given our experience, I am skeptical of their statements :).

karl