libusb: Apple is a POSIX system

externals/libusb/CMakeLists.txt

@@ -1,4 +1,4 @@
-if (MINGW OR (${CMAKE_SYSTEM_NAME} MATCHES "Linux"))
+if (MINGW OR (${CMAKE_SYSTEM_NAME} MATCHES "Linux") OR APPLE)
     set(LIBUSB_FOUND ON CACHE BOOL "libusb is present" FORCE)
     set(LIBUSB_VERSION "1.0.24" CACHE STRING "libusb version string" FORCE)