gvc has made some recent changes that rename its user options
following meson guidelines for GNOME packages[0].
The options used in gvc as a subproject have been renamed
accordingly.
[0] https://wiki.gnome.org/Initiatives/GnomeGoals/MesonPorting
It turns out that NetworkManager does export the directory as pkg-config
variable after all, so use that instead of building the path ourselves
from the prefix.
https://bugzilla.gnome.org/show_bug.cgi?id=789811
Auto-detect options add some convenience for platforms where a
particular feature isn't available - systemd on non-Linux OS comes
to mind - but the downside is that it is easy to accidentally build
without a desired feature. We consider the latter much more serious
nowadays, so turn our auto-detect options into regular boolean
options.
https://bugzilla.gnome.org/show_bug.cgi?id=791007
Meson options are typed, so the prefix isn't necessary, and indeed
looks odd when used to disable the option:
-Denable_foo=false
Also replace underscores with dashes, which is the preferred meson
style.
https://bugzilla.gnome.org/show_bug.cgi?id=791007
Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229
