muscle: factor the kind check in M4

* data/bison.m4 (b4_percent_define_check_kind): New.
Use it to check api.token.prefix.
* data/c++.m4: Check the kind of api.namespace.
* doc/bison.texi: Update a reference to former 'namespace' variable.
* tests/input.at ("%define" code variables): Check api.namespace.