libnm doesn't only search for plugins in the regular VPN plugin directory,
but also in the legacy location and the directory pointed to by the
NM_VPN_PLUGIN_DIR environment variable (if set).
We don't monitor the additional directories, so it's possible for our cache
to become outdated.
Instead of trying to play catch-up with libnm's internals, do what nm-applet
does and use the appropriate API to look up the plugin on each request.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/2386
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
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
