Separate support libraries for REN_CLASSLIB into independent files

[hostilefork]

Currently Rencpp is built with support for std::string and/or Qt. This means "the library" is built with this support. Is it possible or desirable to build the parts that depend on std::string or Qt into a separate library to avoid a link dependency or having to build multiple copies of the main library?

So that would mean instead of rencpp.a as a monolith, you'd have rencpp.a as a slightly smaller library with accompanying rencpp-qt.a and rencpp-std.a libs you could build.