Don't define SDL_ELF_NOTE_DLOPEN when compiler does not support variadic macro's

[madebr]

I removed the cmake comment in the documentation since I feel CMake does not belong in the public headers.

Projects can make sure their toolchain supports SDL_ELF_NOTE_DLOPEN through this fragment:

find_package(SDL3 REQUIRED COMPONENTS Headers)
set(CMAKE_REQUIRED_LIBRARIES SDL3::Headers)
check_c_source_compiles([==[
    #include <SDL3/SDL_dlopennote.h>
    SDL_ELF_NOTE_DLOPEN("sdl-video",
        "Support for video through SDL",
        SDL_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED,
        "libSDL-1.2.so.0", "libSDL-2.0.so.0", "libSDL3.so.0"
    );
    int main(int argc, char *argv[]) {
      return argc + argv[0][1];
    }
  ]==] COMPILER_SUPPORTS_SDL_ELF_NOTE_DLOPEN)

We document how to disable the macro. CMake/meson developers should know what do do with that.

Here's how to test without using the SDL3 headers:

include(CheckCSourceCompiles)
check_c_source_compiles([==[
    #ifndef __ELF__
      ELF DL notes are only supported on ELF platforms
    #endif
    #define ARR1(N1) "[\"" N1 "\"]"
    #define ARR2(N1, N2) "[\"" N1 "\",\"" N2 "\"]"
    #define ARR_GET(N1,N2,NAME,...) NAME
    #define ARR(...) ARR_GET(__VA_ARGS__, ARR2, ARR1)(__VA_ARGS__)
    #define JOIN2(A,B) A##B
    #define JOIN(A,B) JOIN2(A,B)
    #define UNIQ JOIN(s_global, __LINE__)
    #define DLNOTE_INT2(JSON,VARNAME) \\
      __attribute__ ((used,aligned(4),section(".note.dlopen"))) static const struct { \\
        struct { int a; int b; } v; __attribute__((aligned(4))) char dlopen_json[sizeof(JSON)]; \\
      } VARNAME = { { 4, 0x407c0c0aU }, JSON }
    #define DLNOTE_INT(JSON,VARNAME) DLNOTE_INT2(JSON,VARNAME)
    #define DLNOTE(P,...) DLNOTE_INT("[" P "," ARR(__VA_ARGS__) "]", UNIQ)
    DLNOTE("required", "liba.so.0");
    DLNOTE("recommended", "libb.so.0", "libb.so.1");
    int main(int argc, char *argv[]) {
      return argc + argv[0][1];
    }
  ]==] COMPILER_SUPPORTS_ELFNOTES)

Description

Existing Issue(s)

Closes #14214

[slouken]

Suggested change

	"\",\"soname\":" SDL_ELF_JSON_ARRAY(__VA_ARGS__) "}]", \
	"\",\"soname\":" SDL_DLNOTE_JSON_ARRAY(__VA_ARGS__) "}]", \

[slouken]

Projects can make sure their toolchain supports SDL_ELF_NOTE_DLOPEN through this fragment:

This needs to be documented somewhere...

[madebr]

This needs to be documented somewhere...

I reinstated it, but the test is simply trying to use the SDL macro and sets the macro if the test failed.

[slouken]

This needs to be documented somewhere...

I reinstated it, but the test is simply trying to use the SDL macro and sets the macro if the test failed.

It doesn't have to be in the header, it can be in a README, it just needs to be referenced here. Feel free to put the real test in the README. Maybe we need README-dlnotes.md? :)