cmake: do not call clean() when using pgk-config

Change-Id: Ie04f2173743d9403530441a1fab02b3bc4d21b09
Reviewed-on: http://gerrit.aldebaran.lan:8080/17083
Tested-by: gerrit
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>
Tested-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>