added GitHub Action CI & tests for atomicity

[TheTechsTech]

• modifications with additional macros for naming compatibility
• added generic cmakelists.txt build file

[mackron]

Thanks. A proper test for atomicity is a valuable contribution, and something badly lacking for this library. However, I cannot merge this change in its current state. For starters all whitespace and styling changes need to be reverted (including the position of curly braces). After that I can give it a proper review.

Regarding the CMake file, I'm happy enough to add this, however I noticed that you've disabled some warnings:

if(UNIX)
    set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -g")
    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -O3 -fomit-frame-pointer -Wno-return-type")
else()
    add_definitions("/wd4244 /wd4267 /wd4033 /wd4715")
endif()

Are these warnings from c89atomic.h, or the tests? Would you be able to show me a list so I can fix them properly? If they're from your new tests I'd rather they just be fixed naturally in the code than just silenced through the build system.

Another thing, and this is not something that would stop me from merging, but for the thread.c/h files, my preference would be to just use my c89thread library. The reason I say that is just I'm just so tired of maintaining cross-platform threading stuff and I'd rather just use an easy-to-integrate library instead. That's not a major issue though - I can change that later.

[TheTechsTech]

The CMake file has no actual warning, it's an setup I used before, just forgot to remove the whole section.

The thread.c/h can be replaced, I don't use it in any project, it's just here for usage in atomicity.c test only.

I'm using this header file in an fork https://github.com/zelang-dev/rpmalloc, rpmalloc is lock-free using C11 atomics, had to make many bug fixes to have it work with Tiny C Compiler. The GitHub Actions CI here and atomicity.c is just copied from there.

I use VS Code on Windows, I need to modify the auto formate style configurations. I will type fixing that.