Prefer '%define api.value.type' to '#define YYSTYPE', so that Bison
know the type.

Use '%code requires' to declare what is needed to define the api.value.type
(that code is output in the generated header before the generated
definition of YYSTYPE).

Prefer '%define api.prefix' inside the grammar file to '-p' outside,
as anyway the functions defined in the file actually use this prefix.

Prefer `%param` to both `%parse-param` and `%lex-param`.

Closes GH-5138

The annotation %empty is properly enforced: warnings when it's
missing, and errors when it's inappropriate.  Support for %empty was
introduced in Bison 3.0.

Pass -Wempty-rule to Bison.

Closes GH-5134

%pure-parser is deprecated. In our case there is no difference between
true & full, as we don't use locations.

With Bison 3.0 some directives are deprecated:
- %name-prefix "x" should be %define api.prefix {x}
- %error-verbose should be %define parse.error verbose

Bison 3.3 also started emiting more warnings and since PHP souce parsers
are not POSIX compliant this patch fixes this as pointed out via
495a46aa1dc564656bf919cb49aae48a31ae15f4.

The parser files for phpdbg are generated by bison from the *.y file.
Parser files in Zend already follows such approach of these files being
ignored from tracking in the Git repository and they are shipped via
the release packages. This way the end user still don't need to have
bison dependency installed to install PHP.

The genfiles script was refactored to generate the phpdbg parser and lexer
files.

Empty comment in phpdbg parser y template file has been changed to the
YACC compliant /* empty */ instead of custom one.

Since PHP 5.3 flex lexer has been replaced with re2c. Commit
0f9e2b1753661afe1c0dee6982e161fcf00d349f made PHP_PROG_LEX macro still
available for BC.

In commit df6bd506d492292ef4353b0f8da0c34eeb076be5 it was updated. Since
this macro is entirely not used in PHP source code anymore from PHP 5.3
and up, this patch removes it together with some old traces of warnings
suppression and comments.