fixed CMake 'libusb-1.0' not found error under Debian

Under Debian

  $ lsb_release -a

  Distributor ID:	Debian
  Description:	Debian GNU/Linux 7.0 (wheezy)
  Release:	7.0
  Codename:	wheezy

the CMake build script fails with an error "package 'libusb-1.0' not found"

.../rtl-sdr/build $ cmake ../

  ...
  -- Detecting C compiler ABI info - done
  -- Build type not specified: defaulting to release.
  -- Found PkgConfig: /usr/bin/pkg-config (found version "0.26")
  -- checking for module 'libusb-1.0'
  --   package 'libusb-1.0' not found
  -- libusb-1.0 not found.
  -- Looking for include file pthread.h
  ...

The root cause of this error was due to the CMake script 'FindLibUSB.cmake'
looking for 'libusb-1.0' whereas under Debian it was named 'libusb'.

The CMake script has been expanded so that it also works with these additional
Debian specific names.