* src/scan-skel.l: Postprocess quadrigraphs.

* src/reader.c (copy_character): New function, used to output
single characters while replacing `[' and `]' with quadrigraphs, to
avoid troubles with M4 quotes.
(copy_comment): Output characters with copy_character.
(read_additionnal_code): Likewise.
(copy_string2): Likewise.
(copy_definition): Likewise.
* tests/calc.at: Exercise M4 quoting.

tests/calc.at

@@ -67,6 +67,8 @@ typedef int value_t;
 
 %}
 
+/* Exercise M4 quoting: '@:>@@:>@', 0.  */
+
 /* Also exercise %union. */
 %union
 {
@@ -76,6 +78,9 @@ typedef int value_t;
 /* Exercise post-prologue dependency to %union.  */
 %{
 static void id (YYSTYPE *lval);
+
+/* Exercise quotes in declarations.  */
+char quote[] = "@:>@@:>@,";
 %}
 
 /* Bison Declarations */
@@ -83,6 +88,9 @@ static void id (YYSTYPE *lval);
 %token <ival> NUM "number"
 %type  <ival> exp
 
+/* Exercise quotes in strings.  */
+%token "fake @>:@@>:@,"
+
 %nonassoc '=' /* comparison	       */
 %left '-' '+'
 %left '*' '/'
@@ -100,9 +108,14 @@ input:
 
 line:
   '\n'
-| exp '\n' {}
+| exp '\n'
+  {
+    /* Exercise quotes in braces.  */
+    char tmp[] = "@>:@@:>@,";
+  }
 ;
 
+/* Exercise M4 quoting: '@:>@@:>@', 1.  */
 exp:
   NUM                { $$ = $1;             }
 | exp '=' exp
@@ -124,6 +137,7 @@ exp:
 /* The input. */
 FILE *yyin;
 
+/* Exercise M4 quoting: '@:>@@:>@', 2.  */
 static void
 yyerror (const char *s)
 {