refix the librt check for mingw/win/apple (missed the problem building only on lucid).
refix the librt check for mingw/win/apple (missed the problem building only on lucid).

jack-oquin/catkin,beni55/catkin,thaddeustaylor/catkin,shyamalschandra/catkin,shyamalschandra/catkin,jack-oquin/catkin,meyerj/catkin,meyerj/catkin,beni55/catkin,ros/catkin,meyerj/catkin,harmishhk/catkin,beni55/catkin,asmodehn/catkin,wkentaro/catkin,asmodehn/catkin,shyamalschandra/catkin,jack-oquin/catkin,wkentaro/catkin,harmishhk/catkin,ros/catkin,scpeters/catkin,harmishhk/catkin,ros/catkin,thaddeustaylor/catkin,scpeters/catkin,asmodehn/catkin,thaddeustaylor/catkin,scpeters/catkin,wkentaro/catkin