host: Fixed FindLibUSB.cmake issues on OSX Mavericks, cmake touch-ups

Fixed Issue #242, where the FindLibUSB.cmake script wasn't finding a
libusb installed to /usr/local/ in the case where pkg-config wasn't
installed. This should also help in cases where pkg-config isn't
installed on Linux systems.

Other touch-ups include removal of an unnecessary warning message,
and some re-arrangement of operations.