use libusb's pkgconfig support

This streamlines sick_tim's libusb detection.
On Sat, Aug 30, 2014 at 02:40:20AM -0700, Jochen Sprickerhof wrote:
> > +find_package(PkgConfig REQUIRED)
> > +pkg_check_modules(libusb libusb-1.0)
> 
> Why do we need libusb, shouldn't libusb-1.0 be enough?

You don't need libusb and this line doesn't look for it.
libusb is the variable prefix for PkgConfig.

See http://www.cmake.org/cmake/help/v3.0/module/FindPkgConfig.html?highlight=pkg_check_modules