collectd: fix postgresql library detection

Explicitly link against -lpthread and -lm.

Fixes ([1]):
   checking for PQconnectdb in -lpq... no
   checking for PQserverVersion in -lpq... no
   postgresql  . . . . . no (dependency error)

[1] http://autobuild.buildroot.org/results/926/926a43b8f635790d7e9abdc977ea803ddaf8a523/



[Thomas:
  - Rebase on top of master
  - Use += instead of =, which will avoid mistakes in the future.]

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>