What's actually needed here is an explaination of how the license
applies along with the explanation where to find the full text.

Also, the libnm documentation was lacking the licensing information
altogether. Fix fixes it too.

With glib < 2.51.3, gdbus-codegen does not understand "--output-directory" [1].
Hence, the generated files are like

    "build/dbus-org.freedesktop.NetworkManager.Device.WifiP2P.xml"

instead of

    "build/introspection/dbus-org.freedesktop.NetworkManager.Device.WifiP2P.xml"

But gnome.gdbus_codegen() returns a path as if it would be inside
"build/introspection". Hack around that, by patching the correct path
otherwise. This is still ugly, because repeated "ninja -C build" calls
will always try to rebuild this target (because the wrong file name
is considered).

See also [2].

[1] https://gitlab.gnome.org/GNOME/glib/commit/ee09bb704fe9ccb24d92dd86696a0e6bb8f0dc1a
[2] https://github.com/mesonbuild/meson/blob/2e93ed58c30d63da8527ff16375ff9e0642e7533/mesonbuild/modules/gnome.py#L1170

(cherry picked from commit ad9e5995e144c1a123ed1ba7a5f7fde54a83fc73)

Add missing trailing commas that avoids getting noise when another
file/parameter is added and eases reviewing changes[0].

[0] https://gitlab.gnome.org/GNOME/dconf/merge_requests/11#note_291585

We need to copy all introspection files to the same directory when
building the documentation.

Note that we only require Meson 0.44, but for the documentation at
least 0.46 is needed because of a new functionality of
gnome.gdbus_codegen(). In this way we can still build on Travis CI
(without documentation).

nm_settings_docs is only defined with enable_introspection.

The strings holding the names used for libraries have also been
moved to different variables. This way they would be less error
as these variables can be reused easily and any typing error
would be quickly detected.

gtkdoc uses some custom generated targets as content files. However,
there are still two problem. The first is that gtkdoc does not
support targets which are not strings. This is being fixed in the
following issue:

https://github.com/mesonbuild/meson/pull/2806

The second issue is that the gtkdoc function produces a target which
is triggered at install time. This makes the dependencies generation
to not be triggered.

This patch uses a workaround for that second issue.

https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00079.html

The install directories of those targets that match the default
install directories have been removed because they are redundant.

This also allows a simple meson build files and it is unnecessary
to create some paths.

https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00078.html

Documentation was not working in meson due to problems with files
generated in `libnm`. To avoid these problems, workarounds were
used. This problems have been recently fixed so these workarounds
are not necessary anymore.

https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00061.html

meson is a build system focused on speed an ease of use, which
helps speeding up the software development. This patch adds meson
support along autotools.

[thaller@redhat.com: rebased patch and adjusted for iwd support]

https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00022.html