Add :system-include-type option to use GCC C++ headers

[slburson]

The LibTorch C++ distribution is built against the GCC C++ headers. So to wrap it, I needed Claw to use those headers instead of Clang's. (Lots of system types have different mangled names between the two sets of headers — ostream is the one I ran into first, but there turned out to be many more.) I got the effect I wanted by commenting out the first cond clause in claw.util:list-system-paths, but I thought there should be a defwrapper option to do this. (If I already knew what the correct list of paths was, I could supply it as :system-includes, but it's hard to find out what it should be without reading the code that normally generates it.)

So I've added an option to defwrapper named :system-include-type that can be set to :clang or :gcc. It defaults to :default, which produces the current behavior (try Clang first, then GCC if Clang is not available); if it's :clang and Clang is not available, an error is signalled.