clang + libc++ does not work, it still requires libc++_shared.so
runtime even if -lstdc++ is used, which supposed to link with static
version of libc++.

With androideabi-4.9, android build now supports threading.

Android does not have _Exit.  We detect this and use _exit instead.
clang-3.4 has an issue around undefined reference to
__atomic_fetch_add_4, so we stick to gcc-4.8 for now.