cmake: convert spaces in USB_CFLAGS to ';'

cmake/sdlchecks.cmake

@@ -1019,6 +1019,7 @@ macro(CheckUSBHID)
     set(SDL_JOYSTICK_USBHID 1)
     file(GLOB BSD_JOYSTICK_SOURCES ${SDL3_SOURCE_DIR}/src/joystick/bsd/*.c)
     list(APPEND SOURCE_FILES ${BSD_JOYSTICK_SOURCES})
+    separate_arguments(USB_CFLAGS)
     target_compile_options(sdl-build-options INTERFACE ${USB_CFLAGS})
     list(APPEND SDL_EXTRA_LIBS ${USB_LIBS})
     set(HAVE_SDL_JOYSTICK TRUE)