syncline: one line is enough

So far we were issuing two lines for each syncline change:

  /* Line 356 of yacc.c  */
  #line 1 "src/parse-gram.y"

This is a lot of clutter, especially when reading diffs, as these
lines change often.  Fuse them into a single, shorter, line:

  #line 1 "src/parse-gram.y" /* yacc.c:356  */

* data/bison.m4 (b4_syncline): Issue a single line.
Comment improvements.
(b4_sync_start, b4_sync_end): Issue a shorter comment.
* data/c++.m4 (b4_semantic_type_declare): b4_user_code must be
on its own line as it might start with a "#line" directive.

data/c++.m4

@@ -119,7 +119,8 @@ m4_define([b4_semantic_type_declare],
 [    /// Symbol semantic values.
 m4_ifdef([b4_stype],
 [    union semantic_type
-    {b4_user_stype
+    {
+b4_user_stype
     };],
 [m4_if(b4_tag_seen_flag, 0,
 [[    typedef int semantic_type;]],