Consistent builds

Hiya! First off thanks a bunch for putting this together - it's been very useful.

Couple additions for you based on some issues I hit, mainly in that x265 master dropped some params from their api in https://patches.videolan.org/patch/4600/ causing the static build to fail when ffmpeg referenced them. To fix this in a more general way I took the opportunity to define some stable versions to build against when pulling from these external repos, hopefully to keep things stable between your updates.

I had to use git hashes as x264 and rtmpdump don't seem to tag releases.