Stop the C++ confusion in docs

Looking at https://wayland.pages.freedesktop.org/weston/genindex.html, it is full of "C++" when libweston and weston are pure C.

However, this could be unfixable, see the discussion at !182 (comment 175667) about why the primary domain is set to C++. So probably this needs to be fixed in Breathe and/or Doxygen as well.

When I insert links to documented macros, I have to use :c:macro:FOO and :c:func:TEST because the default domain is cpp instead.