embed: Fix GtkClutterEmbed:use-layout-size API doc

See merge request GNOME/clutter-gtk!1

The documentation was cut'n'paste from somewhere else, and didn't even
match in terms of types.

Avoid a compiler warning.

Declare _gtk_clutter_embed_set_child_active() in a shared private
header, so we can avoid a compiler warning.

The Autotools build should do what the Meson build already does, and
keep the interface age in sync with the version.

Changes since 1.8.2:
        • Add Meson build, in parallel with Autotools
        • Fix cruft in the Autotools build
        • Fix a snafu in the name of the getter for the
          GtkClutterEmbed:use-layout-size property; the ABI is preserved

List of bugs fixed:
        #783431 - Cannot build clutter-gtk with meson on FreeBSD because
                  -D_XOPEN_SOURCE=500 is too old

Defining _XOPEN_SOURCE to 500 on FreeBSD means a C89 environment with
_POSIX_C_SOURCE set to 199506, which disables C99 functions. Since
ceilf is a new function added in C99, we should set _XOPEN_SOURCE to a
higher value.

https://bugzilla.gnome.org/show_bug.cgi?id=783431

The installed headers should go in clutter-gtk-1.0/clutter-gtk.

We depend on gobject-introspection at configure time, and have been for
years. Using the internal copy of introspection.m4 means we don't get
eventual bug fixes and changes from upstream.

Instead of our custom m4 macros, so that we can share the same behaviour
between Autotools and Meson builds.

This allows building a dist tarball using Meson.

Meson is a meta-build system that has various advantages over Autotools:

 - it is fast
 - it is portable
 - it is easy to understand
 - it is well maintained

Various projects in the GNOME stack already moved to Meson, including
GTK+.

The documented get_use_layout_size() getter method is still named
get_honor_stage_size() in the library API, even though it does not match
the setter, and it's undocumented and undeclared.

Let's preserve the ABI compatibility, and keep exporting the symbol, but
also let's define the appropriate getter, to avoid an undefined symbol
when developers use it.

If we build the examples in a build directory we still want to be able
to access the ancillary data files.

Instead of using libtool, we should explicitly annotate the exported
symbols in the library API.

GTK-Doc exposes the various package metadata as entities to be included
already.

There's no point in having the reference in its separate directory, as
we don't ship any other documentation.

Modernise the build system just a bit.

This started causing problems with Gtk+ 3.21. As the documentation explains,
disabling double buffering might trigger rendering issues.

https://bugzilla.gnome.org/show_bug.cgi?id=769097

https://bugzilla.gnome.org/show_bug.cgi?id=767627

https://bugzilla.gnome.org/show_bug.cgi?id=759375

We depend on a behavior change in GdkOffscreenWindow for HiDpi
displays.

We need to set the scale factor correctly on the embedded Gtk+ widgets to
avoid blurry widgets on hidpi displays.

https://bugzilla.gnome.org/show_bug.cgi?id=758937