Rename SDL_SDK -> SDL2DIR as it is said in FindSDL2 documentation

CI/before_script.msvc.sh

@@ -557,12 +557,9 @@ printf "SDL 2.0.3... "
 		eval 7z x -y SDL2-2.0.3.zip $STRIP
 	fi
 
-	SDL_SDK="`real_pwd`/SDL2-2.0.3"
-	add_cmake_opts -DSDL2_INCLUDE_DIR="$SDL_SDK/include" \
-		-DSDL2MAIN_LIBRARY="$SDL_SDK/lib/x$ARCHSUFFIX/SDL2main.lib" \
-		-DSDL2_LIBRARY_PATH="$SDL_SDK/lib/x$ARCHSUFFIX/SDL2.lib"
+	export SDL2DIR="`real_pwd`/SDL2-2.0.3"
 
-	add_runtime_dlls `pwd`/SDL2-2.0.3/lib/x$ARCHSUFFIX/SDL2.dll
+	add_runtime_dlls $SDL2DIR/lib/x$ARCHSUFFIX/SDL2.dll
 
 	echo Done.
 }