update prepare.sh

prepare.sh

@@ -64,15 +64,15 @@ ninja -C out/Static
 FFMPEG_BUILD_DIR=${EXTERNAL_BUILD_DIR}/ffmpeg
 FFMPEG_DIR=${EXTERNAL_DIR}/ffmpeg
 
-cd $FFMPEG_DIR
-./configure --disable-programs --enable-shared
-make -j install prefix=${FFMPEG_BUILD_DIR}/dist
-
 if [ ! -d "${FFMPEG_BUILD_DIR}" ]; then
     mkdir ${FFMPEG_BUILD_DIR}
 else
     rm -rf ${FFMPEG_BUILD_DIR}/*
 fi
 
+cd $FFMPEG_DIR
+./configure --disable-programs --enable-shared
+make -j install prefix=${FFMPEG_DIR}/dist
+
 cp -r ${FFMPEG_DIR}/dist/include ${FFMPEG_BUILD_DIR}/
 cp -r ${FFMPEG_DIR}/dist/lib     ${FFMPEG_BUILD_DIR}/