CMake: fix source_group with no src dir, or when files above CMakeLis… #3859

	# Just build documentation using Doxygen - no CUDA required
	name: Docs

	on:
	# Branch pushes that do not only modify other workflow files
	push:
	branches:
	- '**'
	paths:
	- "**"
	- "!.github/**"
	- ".github/workflows/Docs.yml"
	# Disabled for now. See https://github.com/FLAMEGPU/FLAMEGPU2/pull/644
	# pull_request:
	# Allow manual invocation.
	workflow_dispatch:

	defaults:
	run:
	shell: bash

	jobs:
	doxygen:
	runs-on: ubuntu-20.04
	env:
	# Define constants
	BUILD_DIR: "build"
	steps:
	- uses: actions/checkout@v3

	- name: Install doxygen >= 1.9.0 + other dependencies
	run: \|
	# Install graphviz + dependencies to build doxygen from source
	sudo apt -y install graphviz flex bison
	# Download doxygen and build from source
	wget --content-disposition https://github.com/doxygen/doxygen/archive/refs/tags/Release_1_9_1.tar.gz
	tar -zxf doxygen-Release_1_9_1.tar.gz
	mkdir -p doxygen-Release_1_9_1/build
	cd doxygen-Release_1_9_1/build
	cmake .. -DCMAKE_BUILD_TYPE=Release
	cmake --build . -j `nproc`
	echo "$(pwd)/bin" >> $GITHUB_PATH

	- name: Configure cmake
	run: >
	cmake . -B "${{ env.BUILD_DIR }}"
	-Werror=dev
	-DCMAKE_WARN_DEPRECATED="OFF"
	-DFLAMEGPU_WARNINGS_AS_ERRORS="ON"
	-DFLAMEGPU_BUILD_API_DOCUMENTATION="ON"

	- name: Docs
	working-directory: ${{ env.BUILD_DIR }}
	run: cmake --build . --target docs --verbose -j `nproc`

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