Buildbot revealed that Collectd 5.7.1 uses libcap to evaluate system
capabilities and display advice on some plugins (dns, iptables, exec,
ping, ceph, turbostat).

Disable the check in configure.ac in order to avoid a dependency
for libcap. (It is not that big, only 6 kB .ipk, but still avoid that.)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>