fix building on IRIX with GCC

It's impossible to know what problem this was supposed to fix (it's been there
since the Dawn of Time: r2), but it prevents stddef.h from defining ptrdiff_t,
among others. We need ptrdiff_t.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76488 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/platform.h

@@ -321,10 +321,8 @@
 
 #    define __UNIX_LIKE__
 
-    /* Helps SGI compilation, apparently */
 #    ifdef __SGI__
 #        ifdef __GNUG__
-#            define __need_wchar_t
 #        else /* !gcc */
             /*
                Note I use the term __SGI_CC__ for both cc and CC, its not a good