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
