Code review: avidemux patch to use system libass

[Richard Shaw]

I think I've managed to unbundle libass from avidemux but would like
another set of eyes on my patch[1] before submitting a build and
closing the bug report[2[.

I'm not much of a programmer and know even less about cmake so I
pretty much fumbled my way through it. Any comments are welcome.

Thanks,
Richard

[1] https://bugzilla.rpmfusion.org/attachment.cgi?id=631
[2] https://bugzilla.rpmfusion.org/show_bug.cgi?id=506