c++: use // comments in the output

This is mostly used for the license header, the synclines, and the
generated tables:

-  /* STOS_[STATE-NUM] -- The (internal number of the) accessing
-     symbol of state STATE-NUM.    */
+  // STOS_[STATE-NUM] -- The (internal number of the) accessing
+  // symbol of state STATE-NUM.
   static const unsigned char yystos_[];

* data/c.m4: Comment changes.
(b4_comment_): Expand the text argument.
Before this change, we were actually formatting M4 code as a
C comment, and then expand it.
(b4_comment): Fix the closing of comments: there is no reason to
add the (line) prefix before the closing "*/".
* data/c++.m4 (b4_comment): New.

data/c++.m4

@@ -19,6 +19,13 @@
 
 m4_include(b4_pkgdatadir/[c.m4])
 
+# b4_comment(TEXT, [PREFIX])
+# --------------------------
+# Put TEXT in comment. Prefix all the output lines with PREFIX.
+m4_define([b4_comment],
+[b4_comment_([$1], [$2// ], [$2// ])])
+
+
 ## ---------------- ##
 ## Default values.  ##
 ## ---------------- ##

data/c.m4

@@ -58,20 +58,17 @@ m4_define([b4_cpp_guard_close],
 # from OPEN.  That's why we don't patsubst([$1], [^\(.\)], [   \1]).
 #
 # Prefix all the output lines with PREFIX.
-m4_define([b4_comment_], [$2[]m4_bpatsubst([$1], [
+m4_define([b4_comment_],
+[$2[]m4_bpatsubst(m4_expand([$1]), [
 \(.\)], [
 $3\1])$4])
 
 
 # b4_comment(TEXT, [PREFIX])
 # --------------------------
-# Put TEXT in comment.  Avoid trailing spaces: don't indent empty lines.
-# Avoid adding indentation to the first line, as the indentation comes
-# from "/*".  That's why we don't patsubst([$1], [^\(.\)], [   \1]).
-#
-# Prefix all the output lines with PREFIX.
+# Put TEXT in comment.  Prefix all the output lines with PREFIX.
 m4_define([b4_comment],
-[b4_comment_([$1], [$2/* ], [$2   ], [$2  */])])
+[b4_comment_([$1], [$2/* ], [$2   ], [  */])])
 
 
 # b4_identification