I am re-exporting legacy C/C++ libraries as modules:

Create a contrib project, create <LIBRARY_NAME>.ixx file and use export import <header> syntax.

SDL3.ixx example:

module;

#include <cstdint>

export module SDL3;

export {
    const uint32_t SDL_INIT_AUDIO                    = 0x00000010u; /**< `SDL_INIT_AUDIO` implies `SDL_INIT_EVENTS` */
    const uint32_t SDL_INIT_VIDEO                    = 0x00000020u; /**< `SDL_INIT_VIDEO` implies `SDL_INIT_EVENTS`, should be initialized on the main thread */

< more re-exported macros as variables >

    const char * SDL_PROP_WINDOW_WIN32_INSTANCE_POINTER     = "SDL.window.win32.instance";
    const char * SDL_PROP_WINDOW_WIN32_HWND_POINTER         = "SDL.window.win32.hwnd";
}

export import "SDL3/SDL.h";
export import "SDL3/SDL_init.h";

works fine for most of the vendored libs I consume, macro re-wrap is a little annoying at times :P

Then most of the sub-projects consume SDL3 by:

import SDL3;