gdk and gsk are no longer separate libs but part of gtk now, so any
Gtk+ user should just link to gtk, there's no need to additionally
link against all those static helper libs that go into the gtk lib.
This means we need to specifically add confinc to include_directories
in more places to make sure the right config.h (i.e. ours) gets
included and not a subproject's like graphene's config.h.
Not dragging in static libs also fixes the issue of all executables
having to be relinked for any and all changes. With this change
it's super-fast now and can be skipped for most changes that don't
touch the external ABI.
This is how it's done in the autotools build. Also avoids problems
with multiple source files having the same name (gdkeventsource.c).
Also move broadway backend code into broadway subdir.
Add libgdk_dep as dependency to the libgtk_dep declare_dependency(), so
that the generated gdk includes are generated before anything is built
that tries to include gtk headers (such as various tests that don't depend
on gdk directly).
This is needed for the Meson port, a file name .c that's included
and shouldn't be compiled into an object is difficult to manage
otherwise. And it's not actually a valid .c file anyway.
