Add an explicit attribute collapsing specialization for two unused_types

[jktjkt]

The test case is trying to ignore the attribute of the CRLF rule. Without the
patch to make_attribute.hpp, the compiler doesn't know which of the
specializations to use, and the build fails.

[K-ballo]

Could you please reduce the test case to the bare minimum? For instance, you don't need a grammar, a single rule should suffice. Additionally, please make your identifiers all lowercase.

[K-ballo]

I was able to reduce the test case to a minimum:

unused_type attr;
rule<class rule_id> const r;
BOOST_TEST((test_attr("x", r = lit('x'), attr, true)));

Note that it belongs in rule* and not in sequence.

[djowel]

I just migrated X3 to the boost repository. Please have your pull request there. This repo is going away. (https://github.com/boostorg/spirit/tree/develop)