Allow rendering glue code as extern C .cpp file

[keynmol]

If the header you are including is actually a C++ header, then you need to have your glue code rendered as C++ code with extern C, otherwise you get compilation errors.

Also in this mode we need to make sure not to render _Bool - instead render C++ native bool

I think at this point it's better to just add GlueC and GlueCPP to --lang parameter, and just have --scala and --c as shortcuts