Add nghttp2_option_set_recv_client_preface()

By default, nghttp2 library only handles HTTP/2 frames and does not
recognize first 24 bytes of client connection preface. This design
choice is done due to the fact that server may want to detect the
application protocol based on first few bytes on clear text
communication. But for simple servers which only speak HTTP/2, it is
easier for developers if nghttp2 library takes care of client
connection preface.

If this option is used with nonzero val, nghttp2 library checks first
24 bytes client connection preface. If it is not a valid one,
nghttp2_session_recv() and nghttp2_session_mem_recv() will return
error NGHTTP2_ERR_BAD_PREFACE, which is fatal error.
libevent-server: Use nghttp2_option_set_recv_client_preface()
Compile with android NDK r10b 32bit target

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.
Just link to github releases page for released versions