Leave the definition of 'yynmatch' and 'yypmatch' to the user.

Leave the definition of 'yynmatch' and 'yypmatch' to the user.

With '--posix-captures' RE2C stores submatch results in 'yynmatch'
(the total number of capturing groups for the matching rule) and
'yypmatch' (an array of submatch values for each group).

These variables should be user-defined, so that users can override
default implementation (e.g. make 'yypmatch' an array of integer
offsets rather than an array of pointers). Overriding is only possible
with generic API: if default API is used, then RE2C can autogenerate
'yynmatch' and 'yypmatch' (and so it did prior to this commit).
However, it is better to have the same behavior with both APIs; also,
it is coherent with '--tags' option (RE2C leaves tag definition to
the user).