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.  ##
 ## ---------------- ##