make a pure string from a string literal

[Dan Nicolaescu]

There are a few build_string("STRING LITERAL") calls in emacs/src/*.c
The result is actually a constant string, so we could do:
Fpurecopy (build_string ("STRING LITERAL"))
but that creates another copy of "STRING LITERAL" in pure memory, we
already a have a perfectly good one in read only memory.

So how about adding:

Lisp_Object
make_pure_string_from_literal (const char *data)
{
  Lisp_Object string;
  struct Lisp_String *s;

  s = (struct Lisp_String *) pure_alloc (sizeof *s, Lisp_String);
  s->data = data;
  s->size = strlen (data);
  s->size_byte = -1;
  s->intervals = NULL_INTERVAL;
  XSETSTRING (string, s);
  return string;
}

Then we can use make_pure_string_from_literal ("STRING LITERAL"); 

Any objections?
(better suggestions for the function name are welcome)