externals: Work around libusb duplicate GUID errors

Given we have two libraries that seem to use the same identifier, we can
alter one of them so that the variable is used in place, effectively
changing the used identifier, but without altering the source of
libusb.

externals/libusb/CMakeLists.txt

@@ -21,6 +21,9 @@ if(WIN32)
     if (NOT MINGW)
         target_include_directories(usb BEFORE PRIVATE libusb/msvc)
     endif()
+
+    # Works around other libraries providing their own definition of USB GUIDs (e.g. SDL2)
+    target_compile_definitions(usb PRIVATE "-DGUID_DEVINTERFACE_USB_DEVICE=(GUID){ 0xA5DCBF10, 0x6530, 0x11D2, {0x90, 0x1F, 0x00, 0xC0, 0x4F, 0xB9, 0x51, 0xED}}")
 else()
 target_include_directories(usb
     # turns out other projects also have "config.h", so make sure the