Do NOT build tests if Fastor is a dependency

[Bbllaaddee]

I would like to use Fastor as a dependency in a cmake project. To do that, one would probably go for FetchContent module or use a CPM.cmake package.

Obviously, while doing this, I do not want to build Fastor's tests. However, at the moment, Fastor's tests are being built anyways.

Another issue is that currently Fastor exposes BUILD_TESTING option at the top-level CMakeLists.txt file, which might interfere with other projects' own variables.

cmake3.21 introduces PROJECT_IS_TOP_LEVEL option, that allows to check if project is a dependency for other project. There are also presets, but they are far more complex and require more attention.

I propose a simple fix by bumping cmake version to 3.21 from 3.20, adding a condition on Fastor being a top-level project in order to build tests.

My test project for this consists only of two files: main.cpp and CMakeLists.txt.

// main.cpp
#include <iostream>
#include "Fastor/Fastor.h"

int main()
{
	Fastor::Tensor<int,3> a { 1,2,3 };
	Fastor::Tensor<int,3> b { 4,5,6 };
	std::cout << a+b  << std::endl; // so it doesn't compile out
}

# CMakeLists.txt
cmake_minimum_required(VERSION 3.21)

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

project(
    qwe
	LANGUAGES CXX
)

add_executable(main main.cpp)

# ---- Download Fastor ----
include(FetchContent)
FetchContent_Declare(
	fastor
	GIT_REPOSITORY https://github.com/Bbllaaddee/Fastor.git
	GIT_TAG master
)
FetchContent_MakeAvailable(fastor)


target_include_directories(main PUBLIC ${FASTOR_INCLUDE_DIR})
target_link_libraries(main Fastor)