Assign value to LIB_DIR for ninja Linux build.
Assign value to LIB_DIR for ninja Linux build.

Building libvpx on Linux requires knowledge of the static library file.
Assign the directory path to LIB_DIR for ninja.
Review URL: https://chromiumcodereview.appspot.com/10459029
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp