Here I add a few references for Linux.
I did a fresh install today with using a VirtualBox image from osboxes as suggested on Get started with the Elk Software Development Kit!.
I first installed several dependencies:
sudo apt install git build-essential dkms linux-headers-$(uname -r)
sudo apt install curl libcurl4-openssl-dev python-dev libfreetype6-dev pkg-config libx11-dev libxrandr-dev libxinerama-dev libxcursor-dev
Then instead of going one by one, I installed the remaining dependencies as shown in this post:
sudo apt-get install clang freeglut3-dev g++ libasound2-dev libcurl4-openssl-dev libfreetype6-dev libjack-jackd2-dev libx11-dev libxcomposite-dev libxcursor-dev libxinerama-dev libxrandr-dev mesa-common-dev webkit2gtk-4.0
Then, compiling JUCE from the headless_plugin_client_next branch using the Linux Makefile in JUCE/extras/Projucer/Builds/LinuxMakefile with
make CXX=clang++ -j4 worked well.
After working on my VST on JUCE as usual, I downloaded the SDK referred here and added -I/home/osboxes/VST3_SDK/ to Extra Compiler Flags on the exporter.
The plugin then compiled straight away following instructions on VST 2.x Plugins Using JUCE.
The output file was automatically created in this location:
JuceProjectFolder/Builds/LinuxMakefile/build/intermediate/Release/include_juce_audio_plugin_client_VST2_dd551e08.o
Cheers. Stay healthy.