Fixes X11 video backend compilation when no GL is available

For example, in our Raspberry Pi sysroot.

README-raspberrypi.txt

@@ -69,7 +69,13 @@ The final step is compiling SDL itself.
     cd <SDL SOURCE>
     mkdir -p build;cd build
     ../configure --with-sysroot=$SYSROOT --host=arm-raspberry-linux-gnueabihf --prefix=$PWD/rpi-sdl2-installed --disable-pulseaudio --disable-esd
+    make
+    make install
 
+To be able to deploy this to /usr/local in the Raspbian system you need to fix up a few paths:
+    
+    perl -w -pi -e "s#$PWD/rpi-sdl2-installed#/usr/local#g;" ./rpi-sdl2-installed/lib/libSDL2.la ./rpi-sdl2-installed/lib/pkgconfig/sdl2.pc ./rpi-sdl2-installed/bin/sdl2-config
+    
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  Apps don't work or poor video/audio performance
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