Then set a var in link.c for darwin and use that as the conditional?
#elif defined(__APPLE__)
const char *__gnat_run_path_option = "-Wl,-filelist";
const char *__gnat_object_file_option = "";
char __gnat_shared_libgnat_default = STATIC;
int __gnat_link_max = 262144;
unsigned char __gnat_objlist_file_supported = 1;
unsigned char __gnat_using_gnu_linker = 0;
const char *__gnat_object_library_extension = ".a";
unsigned char __gnat_darwin_idiocy = 1;
#else
Well, the name certainly cannot be called __gnat_darwin_idiocy.
Instead, these flags should be tailored by capability, not by platforms.
That would be e.g. "unsigned char __gnat_force_libgcc_option = 1;"