Better use the atexit function

[mr-cheff]

f4c4629#r117789581

[mr-cheff]

Also maybe, add the atexit function each time msinit or msnew is called

[Khhs167]

Destructors work better than atexit as of right now, as they're called automatically.

On top of this, atexit doesn't function in the same way as destructors, as destructors are invoked whenever the library is unloaded, so for dynamic loading via dlopen atexit won't work.

[mr-cheff]

but it's just for gcc, isn't it?

Maybe add the check if it's compiled for gcc in macros or not

[Khhs167]

that's exactly what I'm doing