cpp: define 'sun', like /usr/lib/cpp does, unfortunately

@@ -1580,13 +1580,12 @@
         errorloc=ppsym("error");
         for (i=sizeof(macbit)/sizeof(macbit[0]); --i>=0; )
                 macbit[i]=0;
 
         if (! nopredef) {
-# if unix
         ysysloc=stsym("unix");
-# endif
+        ysysloc=stsym("sun");
 # if __sparc__
         varloc=stsym ("sparc");
 # endif
 # if __i386__
         varloc=stsym ("i386");