Switch to Meson

[Felipe Borges <felipeborges gnome org>]

Hi,

Since we are just past the release 3.26[0], the new 6 months cycle
shall give us plenty of time to test the new build system, and to file
and fix build related bugs.

My planing is to make a gradual transition by merging the meson build
code[1] and keep it working simultaneously with the current autotools
machinery.

At some point in the cycle we should decide whether the meson build is
good enough and finally drop autotools entirely (making jhbuild build
from meson by default, for instance).

This way I encourage you to try the new build with its different
options in order to stress it as much as possible.

A normal Boxes build under meson should require $meson_version >=
0.40, and the following instructions:

    $ meson build
    $ cd build
    $ ninja
    $ ./build/src/gnome-boxes # To run from the build dir
Or
    $ sudo ninja install # To install it against the system

[0] https://git.gnome.org/browse/gnome-boxes/log/?h=gnome-3-26
[1] https://git.gnome.org/browse/gnome-boxes/log/?h=wip/feborges/meson