Build system: Straighten up building of the DF plugin.

In rare cases, e. g. Solaris, getmntent is available but neither the one nor
the two argument version could be recognized. This lead to `cu_mount_getlist'
returning NULL without an error, leading to the df plugin fail without notice.

While at it I re-implemented the dependency checking of the DF plugin to match
the proprocessor logic in utils_mount.c.