prune include directives

[Domagoj Šarić]

I was trying to figure out what was still including boost/foreach.hpp after removing every include of it in my own code, so I used the /showIncludes MSVC compiler option and it turns out it is some "deep" Spirit header finally included by boost/spirit/home/karma/generate.hpp. It also showed that boost/spirit/home/karma/generate.hpp includes an enormous amount of other headers while I include it only for a simple boost::spirit::karma::generate() (with real_generator) call. Please do prune/clean up the included headers in Boost.Spirit (and/or provide more fine grained versions of the current ones)...

[Domagoj Šarić]

The "deep" header is home/support/info.hpp which also includes list, iterator and boost/variant none of which I need (including the info struct itself). It also expects (which is a "fragile includes" bug IMO) <string> to be included which I also actually do not need...