darwin compat fixups

- delete commented out code
 - delete redundant check for missing
   mmacosx-version-min/maxdir
 - add TODO comment in std library
 - rename 'os' to 'self' in io.zig
 - `openSelfExe` aborts on darwin instead of compile error
 - only allow warnings on the one parseh test that has
   `#include <stdint.h>`.

src/main.cpp

@@ -389,16 +389,6 @@ int main(int argc, char **argv) {
                 return EXIT_FAILURE;
             }
 
-            if((g->zig_target.os == ZigLLVM_Darwin ||
-                    g->zig_target.os == ZigLLVM_MacOSX ||
-                    g->zig_target.os == ZigLLVM_IOS) &&
-                (!mmacosx_version_min &&
-                    !mios_version_min &&
-                    !g->mmacosx_version_min &&
-                    !g->mios_version_min) && target) {
-                zig_panic("unable to determine -mmacosx-version-min or -mios-version-min");
-            }
-
             if (mmacosx_version_min) {
                 codegen_set_mmacosx_version_min(g, buf_create_from_str(mmacosx_version_min));
             }