TexturePacker faild to build with no -I/include/dir in NATIVE_CXXFLAGS

...Change ensures
-I. \
-I@abs_top_srcdir@/lib \
-I@abs_top_srcdir@/xbmc \
-I@abs_top_srcdir@/xbmc/linux
when NATIVE_ROOT_PATH=@USE_TEXTUREPACKER_NATIVE_ROOT@ results in "NATIVE_ROOT_PATH=", like happened to me twice, compiled xbmc on RPI...