With Meson, using the `soversion` argument of a library() target means
setting the explicit soname, e.g.:

    soversion: 17.0.2

will be used to generate:

    libgnome-desktop-3.so.17.0.2

Unlike libtool, though, Meson will not generate the symbolic links for
the first component of the soversion:

    libgnome-desktop-3.so.17

Which is what the dynamic linker will actually use to resolve the
library dependency at link time.

In order to get a symbolic link, we need to use the `version` field for
the soname, and the `soversion` field for the first component:

    version: '17.0.2'
    soversion: '17'

To avoid having to manually set two fields, we can generate the
`soversion` value from the `version` one, so that they will always be in
sync.

This fixes the build of gnome-shell on Continuous, which has been
failing since gnome-desktop has been moved to Meson with the error:

    ld: warning: libgnome-desktop-3.so.17, needed by /usr/lib/libmutter-3.so,
    not found (try using -rpath or -rpath-link)

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>