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

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.