avoid direct strncmp calls.

Before this change, bison would accept either .tab and _tab equivalently,
whatever the current platform.  Besides, it was not obeying everywhere
to the possible definition of TAB_EXT to something else than .tab.

For consistency, handle only TAB_EXT (".tab" on non DJGPP platforms).
Support for "_tab" is neither documented, nor tested.

	* src/system.h (STRNCMP_LIT): New.
	From Jim Meyering.
	(STRPREFIX_LIT): New.
	* src/files.c, src/getargs.c: Use it.

src/getargs.c

@@ -102,7 +102,7 @@ flags_argmatch (const char *option,
       args = strtok (args, ",");
       while (args)
         {
-          int no = strncmp (args, "no-", 3) == 0 ? 3 : 0;
+          int no = STRPREFIX_LIT ("no-", args) ? 3 : 0;
           int value = XARGMATCH (option, args + no, keys, values);
           if (value == 0)
             {