Unreliable behavior when using asm

Hi all, As you probably know, the x264 asm support in this fork is quite lacking and sadly it is causing consistent and reliable segfaults on certain CPUs. See my comment on gstreamer/gst-plugins-ugly#36 (moved). I have made a fork of this repo which uses unstable_external_project instead. This has solved my segfault.

I haven't made it a pull request because it sortof defeats the purpose of this port. However, at the same time, it seems more beneficial to do this until all of the #FIXME's are resolved.