cmake: do not cache libusb library if libusb-1.0 headers not found

Library found might be an older libusb version.

Closes #158

Author: maxnet

cmake/Findlibusb-1.0.cmake

@@ -87,6 +87,7 @@ else (LIBUSB_1_LIBRARIES AND LIBUSB_1_INCLUDE_DIRS)
 	  message(STATUS " - Libraries: ${LIBUSB_1_LIBRARIES}")
 	endif (NOT libusb_1_FIND_QUIETLY)
   else (LIBUSB_1_FOUND)
+	unset(LIBUSB_1_LIBRARY CACHE)
 	if (libusb_1_FIND_REQUIRED)
 	  message(FATAL_ERROR "Could not find libusb")
 	endif (libusb_1_FIND_REQUIRED)