On NixOS all packages are installed into their own immutable prefix.
Because of this get_pkgconfig_variable will return a
path from within systemd's prefix and we cannot write to it.
By using define_variable we can replace the respective directory
to be from the paths from meson.
Don't recurse into the directory of systemd when it was not found.
There are still some issues because libudev is actually provided
by systemd these days and we still need that for building the v4l2 and
alsa monitors.
