build-lib emitted headers should be able to depend on system includes

[daurnimator]

const std = @import("std");

const lua = @cImport({
    @cInclude("lua.h");
    @cInclude("lauxlib.h");
});

export fn luaopen_mylib(L: *lua.lua_State) c_int {
    return 0;
}

zig build-lib -dynamic --library c -isystem /usr/include/ src/main.zig

The output .h file contains:

#ifndef MAIN_H
#define MAIN_H


#ifdef __cplusplus
#define MAIN_EXTERN_C extern "C"
#else
#define MAIN_EXTERN_C
#endif

#if defined(_WIN32)
#define MAIN_EXPORT MAIN_EXTERN_C __declspec(dllimport)
#else
#define MAIN_EXPORT MAIN_EXTERN_C __attribute__((visibility ("default")))
#endif

struct struct_lua_State;

MAIN_EXPORT int luaopen_mylib(struct struct_lua_State * L);

#endif

This defines the opaque type struct struct_lua_State, when it needs to #include <lua.h> and use the existing opaque type lua_State.

[andrewrk]

Related: #2041

[andrewrk]

This issue is what @tiehuis proposed in #2041 (comment)