Base conf file path on sysconfdir. sysconfdir can be specified with an
absolute or relative path. To ensure PINOS_CONFIG_DIR is set correctly,
use join_paths() to prepend prefix when sysconfdir is relative.
Meson added support for sysconfdir in version 0.34.0 and join_paths() in
version 0.36.0.
It was sometimes assumed to be 0.1, sometimes 1.0, and sometimes not to
be used at all. This commit makes things consistent and so that it is
possible to link externally.
