Log Message:
lxqt-panel: rework handling of various plugins.

when a plugin's dependency is unavailable in pkgsrc, explicitly disable
it everywhere, rather than just !Linux. when the plugin is tested
as not working on NetBSD, also disable it.

enable network monitor plugin by default, it works fine on NetBSD.

make "alsa" and "pulseaudio" options. Disable pulseaudio by \ 
default
since there's also an oss engine which can control volume natively
on NetBSD. enable alsa on Linux.

should help the build on various platforms.

TODO for me: the same but for lxqt-session