Improve support for C11/atomic

Consider the following c11 code:

_Atomic(long long) ll;

int main(void)
{
        ++ll;
        return 0;
}

If one use the following CMake code:

cmake_minimum_required(VERSION 3.13)
project(p)
add_executable(a a.c)
set_property(TARGET a PROPERTY C_STANDARD 11)

It will fails on some arch supported by Debian (armel, mipsel and powerpc) with something like this:

/usr/bin/ld: CMakeFiles/a.dir/a.c.o: in function `main':
a.c:(.text+0x54): undefined reference to `__atomic_fetch_add_8'
/usr/bin/ld: a.c:(.text+0x5c): undefined reference to `__atomic_fetch_add_8'
collect2: error: ld returned 1 exit status

Previous bug reports have been made in the past:

1. #21174 (closed)
2. #20895 (closed)

The basic idea is that cmake should not unconditionally add -latomic to GCC toolchain. However there is room for improvement to avoid boilerplate code in cmake listing.

It is not clear how it should be solved at this point.

1. I see that this is solved for Android:

% grep -r latomic /usr/share/cmake-3.22/Modules
/usr/share/cmake-3.22/Modules/Platform/Android/ndk-stl-c++_static.cmake:  string(APPEND CMAKE_${lang}_STANDARD_LIBRARIES " -latomic") # provided by toolchain
/usr/share/cmake-3.22/Modules/Platform/Android-Common.cmake:    string(APPEND CMAKE_${lang}_STANDARD_LIBRARIES " -latomic -lm")

2. Let's consider now std::thread (c++11) support in cmake. It is assumed that the programmer understand that std::thread on Linux is implemented using pthread. Thus for the following c++11 program:

#include <iostream>
#include <thread>
#include <mutex>

std::once_flag flag;

void simple_do_once()
{
    std::call_once(flag, [](){ std::cout << "Hello, world!\n"; });
}

int main()
{
    std::thread st1(simple_do_once);
    st1.join();

    return 0;
}

It is expected to write the following CMake code:

cmake_minimum_required(VERSION 3.13)
project(p)

set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
set(THREADS_PREFER_PTHREAD_FLAG TRUE)
find_package(Threads REQUIRED)
add_executable(b b.cxx)
set_property(TARGET b PROPERTY CXX_STANDARD 11)
target_link_libraries(b Threads::Threads) # REQUIRED

What would be the best approach for CMake to provide -latomic :

1. Add it to CMAKE_C_STANDARD_LIBRARIES ?
2. Through a module: find_package(Atomic REQUIRED) ?