diagnostics: improve them for %name-prefix

Currently the diagnostics for %name-prefix are not precise enough.  In
particular, they does not show that braces must be used instead of
quotes.

Before:

    foo.y:3.1-14: warning: deprecated directive, use '%define api.prefix' [-Wdeprecated]
     %name-prefix = "foo"
     ^^^^^^^^^^^^^^

After:

    foo.y:3.1-20: warning: deprecated directive, use '%define api.prefix {foo}' [-Wdeprecated]
     %name-prefix = "foo"
     ^^^^^^^^^^^^^^^^^^^^

To do this we need the value passed to %name-prefix, so move the
warning from the scanner to the parser.

Accuracy will be very important for the forthcoming changes.

* src/parse-gram.y (do_name_prefix): New.
(PERCENT_NAME_PREFIX): Have a semantic value: the raw source, with
possibly underscores, equal sign, and spaces.  This is used to provide
a more accurate message.  It does not take comments into account,
but...
* src/scan-gram.l (%name-prefix): Delegate the warnings to the parser.

* tests/headers.at, tests/input.at: Adjust expectations.

src/parse-gram.y

@@ -43,8 +43,9 @@
   #include "named-ref.h"
   #include "quotearg.h"
   #include "reader.h"
-  #include "scan-gram.h"
   #include "scan-code.h"
+  #include "scan-gram.h"
+  #include "vasnprintf.h"
   #include "xmemdup0.h"
 
   static int current_prec = 0;
@@ -81,6 +82,10 @@
      string from the scanner (should be CODE). */
   static char const *translate_code_braceless (char *code, location loc);
 
+  /* Handle a %name-prefix directive.  */
+  static void do_name_prefix (location const *loc,
+                              char const *directive, char const *value);
+
   /* Handle a %require directive.  */
   static void do_require (location const *loc, char const *version);
 
@@ -190,7 +195,9 @@
 %printer { fputs (quotearg_style (c_quoting_style, $$), yyo); } STRING
 %printer { fprintf (yyo, "{\n%s\n}", $$); } <char*>
 
-%type <uniqstr> BRACKETED_ID ID ID_COLON PERCENT_FLAG TAG tag tag.opt variable
+%type <uniqstr>
+  BRACKETED_ID ID ID_COLON PERCENT_FLAG PERCENT_NAME_PREFIX TAG
+  tag tag.opt variable
 %printer { fputs ($$, yyo); } <uniqstr>
 %printer { fprintf (yyo, "[%s]", $$); } BRACKETED_ID
 %printer { fprintf (yyo, "%s:", $$); } ID_COLON
@@ -307,7 +314,7 @@ prologue_declaration:
       code_scanner_last_string_free ();
     }
 | "%language" STRING            { language_argmatch ($2, grammar_prio, @1); }
-| "%name-prefix" STRING         { spec_name_prefix = $2; }
+| "%name-prefix" STRING         { do_name_prefix (&@$, $1, $2); }
 | "%no-lines"                   { no_lines_flag = true; }
 | "%nondeterministic-parser"    { nondeterministic_parser = true; }
 | "%output" STRING              { spec_outfile = $2; }
@@ -846,6 +853,31 @@ add_param (param_type type, char *decl, location loc)
 }
 
 
+static void
+do_name_prefix (location const *loc,
+                char const *directive, char const *value)
+{
+  spec_name_prefix = value;
+
+  char buf1[1024];
+  size_t len1 = sizeof (buf1);
+  char *old = asnprintf (buf1, &len1, "%s\"%s\"", directive, value);
+  if (!old)
+    xalloc_die ();
+  char buf2[1024];
+  size_t len2 = sizeof (buf2);
+  char *new = asnprintf (buf2, &len2, "%%define api.prefix {%s}", value);
+  if (!new)
+    xalloc_die ();
+  bison_directive (loc, old);
+  deprecated_directive (loc, old, new);
+  if (old != buf1)
+    free (old);
+  if (new != buf2)
+    free (new);
+}
+
+
 static void
 do_require (location const *loc, char const *version)
 {

src/scan-gram.l

@@ -144,7 +144,8 @@ splice   (\\[ \f\t\v]*\n)*
 
 /* An equal sign, with optional leading whitespaces. This is used in some
    deprecated constructs. */
-eqopt    ([[:space:]]*=)?
+sp       [[:space:]]*
+eqopt    ({sp}=)?
 
 %%
 %{
@@ -266,10 +267,7 @@ eqopt    ([[:space:]]*=)?
 
   /* Deprecated since Bison 2.6 (2012-07-19), but the warning is
      issued only since Bison 3.3. */
-  "%name"[-_]"prefix"{eqopt} {
-    deprecated_directive (loc, yytext, "%define api.prefix");
-    return BISON_DIRECTIVE (NAME_PREFIX);
-  }
+  "%name"[-_]"prefix"{eqopt}{sp}    RETURN_VALUE (PERCENT_NAME_PREFIX, uniqstr_new (yytext));
 
   /* Deprecated since Bison 2.7.90, 2012. */
   "%default"[-_]"prec"              DEPRECATED ("%default-prec");