Anyone know how to do this and have the will to make life easier for poor overworked library maintainers?

I have `b2 <stdlib>libc++` on CI for Spirit with https://github.com/boostorg/spirit/blob/develop/.travis.yml#L119-L121 When I tried to make a PR I stuck at the thing that I do not know how (or it is not possible) to format (free-)future value into some flags, i.e:

toolset.flags clang-linux.compile OPTIONS <stdlib> : -stdlib=[ value-of-feature <stdlib> ] ; toolset.flags clang-linux.link OPTIONS <stdlib> : -stdlib=[ value-of-feature <stdlib> ] ;

But since in the wild there are two possible options it might be ok to just hardcode them for now.