Add build.zig

[iacore]

This will make this repo a Zig package.

TODO

• [ ] configurable -DXXXX

[vnmakarov]

I am not well familiar with ZIG (I only read common description of the language and I liked it). I have no time check that this will build mir by zig but I trust you and I am not against to include this PR into repository if/when you finish this.

[iacore]

As of Zig 0.11.0, putting build.zig in this repo is not much useful.

I noticed that when building libmir.a, c2mir/c2mir.c is also included. Why is the file included?

Also, MIR relies on way too much C UB. (this is not a problem in practice.) If you build the project with $CC -fsanitize=undefined you will see where relies on UB.