Make check for empty SDL_MODE environment variable more robust

And here is the next one of my pull requests for my own convenience ;-)
I use homebrew to install SDL as dylibs and always need to set SDL_MODE to dylib manually. Since pkg-config returns zero, this should have to do with the check for SDL_MODE's emptiness (and a little testing seemed to confirm this). The fix was taken from the [GNU make manual page](https://www.gnu.org/software/make/manual/html_node/Text-Functions.html).