Update pl-comp.c (offsetof)

I understand that offsetof is available since C99, so I don't check for it.

src/pl-comp.c

@@ -365,9 +365,7 @@ typedef struct
   unsigned int	entry[1];
 } var_table, *VarTable;
 
-#undef struct_offsetp
-#define struct_offsetp(t, f) ((size_t)((t*)4)->f - (size_t)((t*)4))
-#define sizeofVarTable(isize) (struct_offsetp(var_table, entry) + sizeof(int)*(isize))
+#define sizeofVarTable(isize) (offsetof(var_table, entry) + sizeof(int)*(isize))
 
 #define mkCopiedVarTable(o) copyVarTable(alloca(sizeofVarTable(o->isize)), o)
 #define BITSPERINT (int)(sizeof(int)*8)