syncline: one line is enough

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.