Ignore LIBUSB_FOUND on Apple

It shouldn't be set in the first place, and its cached value is screwing
up the buildbots.

CMakeLists.txt

@@ -738,18 +738,20 @@ if(ENABLE_SDL)
 	endif(SDL2_FOUND)
 endif()
 
-if(NOT APPLE)
+if(NOT ANDROID)
+        add_definitions(-D__LIBUSB__)
+        if(NOT APPLE)
 	        find_package(LibUSB)
-endif()
-add_definitions(-D__LIBUSB__)
-if(LIBUSB_FOUND)
+        endif()
+        if(LIBUSB_FOUND AND NOT APPLE)
 	        message("Using shared LibUSB")
         	include_directories(${LIBUSB_INCLUDE_DIR})
-else()
+        else()
 	        message("Using static LibUSB from Externals")
         	add_subdirectory(Externals/libusb)
 	        set(LIBUSB_LIBRARIES usb)
-endif(LIBUSB_FOUND)
+        endif()
+endif()
 
 set(SFML_REQD_VERSION 2.1)
 if(NOT APPLE)