Revert "MAC_OS_X_VERSION_MAX_ALLOWED is always >= 1080 now"

This reverts commit b02e958fa0bf7d062a5b8aff03d22325e3068feb.

Change-Id: Ia7bf904071ac93f31c999433c74369c56c74c470
Use MACOSX_SDK_VERSION instead of MAC_OS_X_VERSION_MAX_ALLOWED

Change-Id: If24adf787821b2f83a7ecf8806f4d36b9eb7dbdc
Revert another "is always >= 1080" commit

reverted commit: fe25090e992eefd0c43ca73b656632a650793b1a

use MACOSX_SDK_VERSION instead of MAC_OS_X_VERSION_MIN_ALLOWED

Change-Id: Ib6e63e664dc073a8d751306f613e8be4918c406a
use GNU objc ABI/runtime instead of NeXT ABI/runtime when compiler is GCC

with workaround for "cannot find interface declaration for NXConstantString"

also,
  + include <objc/objc-api.h> when OBJC_EXPORT is not defined
  + include <objc/message.h> for objc_msgSend

Change-Id: I5f0df322435be8244d372642dee1c21d783306f1