c++: issue a compile-time warning when #define YYSTYPE is used

Using #define YYSTYPE has always been strongly discouraged in C++.
Macros are dangerous and can result in subtle bugs.
https://lists.gnu.org/r/bug-bison/2020-12/msg00007.html

Maybe some people are currently using #define YYSTYPE.  Instead of
dropping support right now, first issue a warning.  Bison can "see" if
YYDEBUG is defined (it could even be on the command line), only the
compiler knows.  Unfortunately `#warning` is non-portable, and
actually GCC even dies on it when `-pedantic` is enabled.  So we need
to use `#pragma message`.  We must make it conditional as some
compilers might not support it, but it doesn't matter if only _some_
compilers emit the warning: it should be enough to catch the attention
of the developers.

* data/skeletons/c++.m4: Issue a warning when the user defined
YYSTYPE.
* tests/actions.at: Don't #define YYSTYPE.
* tests/headers.at (Several parsers): Ignore the YYSTYPE in the
warning.

tests/actions.at

@@ -1777,8 +1777,6 @@ AT_DATA_GRAMMAR([[input.y]],
     float fval;
   } sem_type;
 
-# define YYSTYPE sem_type
-
 ]AT_CXX_IF([[
 # include <cstdio> // EOF.
 # include <iostream>
@@ -1800,6 +1798,8 @@ AT_DATA_GRAMMAR([[input.y]],
 ]])[
 }
 
+%define api.value.type {sem_type}
+
 %code
 {
   ]AT_YYERROR_DECLARE[