Aded CMake as a build tool

[LinusHenke1701]

I was looking for a nice C/C++ arg parser, where I stumbled upon your repository. I however like to build my projects with CMake and manage my external dependencies with the FetchContent command. I added the necessarry files to do that and build and install it locally by downloading cloning the repo. For local installation a user just has to run

$ mkdir build && cd build
$ cmake ..
$ sudo make install

if the user wants to run tests they have to run

$ mkdir build && cd build
$ cmake -DBUILD_TESTS ..
$ ctest

Similarly you can also build the examples like that. Now if you want to use the library after installation in a project you just have to follow this simple example:

cmake_minimum_required(VERSION 3.16)
project(test)

find_package(args REQUIRED)

add_executable(test main.c)
target_link_libraries(test PRIVATE args)

Or if the library should not be installed globally but only for building a binary one can use FetchContent:

cmake_minimum_required(VERSION 3.16)
project(test)
include(FetchContent)

FetchContent_Declare(
    args
    GIT_REPOSITORY https://github.com/nmulholl/args.git
    GIT_TAG 3.3.1 # This is just a preliminary tag. If you change it, note that you have to change the version info in CMakeLists.txt
)
FetchContent_MakeAvailable(args)

add_executable(test main.c)
target_link_libraries(test args)

Cheers
Linus