Community Builds of Plex Media Player Embedded 2.x for Raspberry Pi 2/3[+] & NUC/x86_64
UPDATE: Plex's embedded Nightlies are being built again :-D
The below is now largely redundant but left for prosperity (and for those who have a Raspberry Pi 3B+).
We haven't had a new release of Plex Media Player Embedded for a while since the good old days of October 2017 and version 1.3.11.. I imagine it's official release is just around the corner, but I really wanted to try out the 2.x series for a project I'm working on. ( I had trouble using the Plex Companion API in 1.3.11. ) Any excuse for the new shiny :-)
The community builds have x86 images available for those on NUC's, whilst the provided Docker and DIY builds are currently just building for the Raspberry Pis.
Remember following anything below is at your own risk and is not supported by anyone in this thread or Plex.
Untested Raspberry Pi 3+ builds from https://github.com/stedaniels/LibreELEC.tv:
web-client 25-fea08608883bd1 and qt9.5.4
Docker build container
DIY build instructions
If you want to build for the Raspberry Pi 3+ please replace "https://github.com/plexinc/LibreELEC.tv plex-LibreELEC.tv" in step 2 with "https://github.com/stedaniels/LibreELEC.tv plex-LibreELEC.tv". I've forked Plex's LibreELEC.tv 8.2.2 build and merged it with the latest LibreELEC.tv build for 8.2.5 which contains Raspberry Pi 3+ support and fixes. I'm currently building this and haven't tested it yet.
Install a fresh copy of Ubuntu 16.04 Server in a VM (left as an exercise for the reader, if you already have Ubuntu feel free to try this yourself) and fully update/upgrade it, generate some ssh keys and add them to Github.
sudo apt update
sudo apt upgrade
Then copy the contents of ~/.ssh/id_rsa.pub to GitHub https://github.com/settings/keys
sudo shutdown -r now
Clone plexinc/LibreELEC.tv and checkout dist-master and do a little fixup for ffmpeg 
git clone https://github.com/plexinc/LibreELEC.tv plex-LibreELEC.tv
git checkout dist-master
sed -i '/--disable-ffserver \\/d' packages/addons/addon-depends/ffmpegx/package.mk packages/multimedia/ffmpeg/package.mk packages/plex/multimedia/ffmpeg-plex/package.mk
Have a read of PlexBuild.
Add the i386 architecture, clean up the cache and install optional and recommended packages.
sudo dpkg --add-architecture i386
sudo apt cleanup
sudo apt update
sudo apt install build-essential wget bc gawk gperf zip unzip lzop xsltproc openjdk-9-jre-headless libncurses5-dev texi2html libexpat1 gcc-multilib libexpat1-dev:i386 libfreetype6-dev:i386 libexpat1-dev libfreetype6-dev fontconfig:i386
If you jump the gun and try build command too early you might get this:
/usr/include/c++/5/iosfwd:38:28: fatal error: bits/c++config.h: No such file or directory
sudo apt install g++-multilib
DISTRO=PlexMediaPlayer PROJECT=RPi2 ARCH=arm PMP_REPO=plex-media-player PMP_BRANCH=master make image
Go have fun for 10-12 hours, your time may vary.
Realise you've actually ended up with 188.8.131.52, woohoo! It's in ./target/
Get it out of the VM and burn it to an SD card, slap it in a RasPi and enjoy :-)
LiveTV works great, movies and episodes, etc. not had chance to test much because I got distracted once Anchorman got going!
A massive thanks to @LongChair and the rest of the Plex team for getting this working
ffmpeg fixup prevents:
BUILD ffmpeg-plex (target) Unknown option "--disable-ffserver".
This is because ffserver seems to have been removed from ffmpeg, the below code will find any other references.
grep -rnwl . -e '--disable-ffserver'