configure.ac: add check for libconfig++
configure.ac: add check for libconfig++

Phosphor-pid-control uses libconfig++ at present for run-time
configuration parsing.  Therefore it should verify its presence.

Change-Id: Ia5819750aa74963746461518786f1dcf6c04a556
Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>

openbmc/phosphor-pid-control,openbmc/phosphor-pid-control