How to do conditional compilation with Zig?

For example, I can add definitions for C/C++ preprocessor with CMake

add_definitions(-DFOO -DBAR ...)

and then I can use them for conditional compilation

#ifdef FOO
   code ...
#endif

#ifdef BAR
   code ...
#endif

Is there a way to do the same thing with Zig and its build system using compilation arguments or something like that?

You can do something similar using the build system. This requires some boilerplate code to do the option handling. Following the tutorial on https://zig.news/xq/zig-build-explained-part-1-59lf for the build system and https://ziggit.dev/t/custom-build-options/138/8 for the option handling:

1. Create a separate file called build.zig that contains a function build():

const std = @import("std");

pub fn build(b: *std.build.Builder) !void {
    const build_options = b.addOptions();
    // add command line flag
    // and set default value
    build_options.addOption(bool, "sideways", b.option(bool, "sideways", "print sideways") orelse false);

    // set executable name and source code
    const exe = b.addExecutable("hello", "hello.zig");
    exe.addOptions("build_options", build_options);
    // compile and copy to zig-out/bin
    exe.install();
}

2. Use the option for conditional compilation in a separate file hello.zig using @import("build_options"):

const std = @import("std");

pub fn main() !void {
    const print_sideways = @import("build_options").sideways;
    const stdout = std.io.getStdOut().writer();
    if (print_sideways) {
       try stdout.print("Sideways Hello, {s}!\n", .{"world"});
    } else {
       try stdout.print("Regular Hello, {s}!\n", .{"world"});
    }
}

3. Compile with:

zig build -Dsideways=true

4. Executing zig-out/bin/hello gives the following output:

Sideways Hello, world!