Updated json module to v.3.11.3. #288

	name: Ubuntu-20.04
	on: [push]

	jobs:
	build:
	strategy:
	matrix:
	compiler: [g++]
	os: [ubuntu-20.04]

	runs-on: ${{ matrix.os }}

	steps:
	- uses: actions/checkout@v1
	- name: submodule
	run: \|
	git submodule init
	git submodule update
	dpkg --list \| grep compiler
	- if: matrix.compiler == 'g++-4.8' && matrix.os != 'macOS-latest'
	name: install gcc 4.8
	run: \|
	# It has become quite tricky to install GCC 4.8 on Ubuntu 20.04.
	# This way does no longer work :(
	sudo apt-get install g++-4.8
	dpkg --list \| grep compiler
	echo "CC=gcc-4.8" >> $GITHUB_ENV
	echo "CXX=g++-4.8" >> $GITHUB_ENV
	- name: Set env clang
	if: matrix.compiler == 'clang'
	run: \|
	echo "CC=clang" >> $GITHUB_ENV
	echo "CXX=clang" >> $GITHUB_ENV
	- name: cmake
	run: mkdir _build && cd _build && cmake ../ -DENABLE_CXX17=OFF
	- name: build
	run: cd _build && cmake --build ./

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