Library code has not changed at all since the last release, so LT
revision stays the same.

Now we don't use std::future with --disable-threads, checking
std::future in configure.ac was removed and building h2load is always
enabled.

The libnghttp2_asio library is C++ library built on top of libnghttp2.
Currently, it has server API and easily create HTTP/2 server using
node.js like API calls.  See the example server source code in
examples/asio-sv.cc.  The library depends on Boost::ASIO library.

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.

Previously we only update consumed flow control window when number of
bytes read in nghttp2 and spdylay callback is 0.  Now we notify
nghttp2 library the consumed bytes even if number of bytes read > 0.
This change also uses newly added spdylay_session_consume() API, so we
require spdylay >= 1.3.0.

libjemalloc as static library requires -pthread flag to link.  Without
that, the check in configure.ac fails with unresolved symbols.

* -Wpragmas
* -Wunreachable-code
* -Wpedantic
* -Waddress
* -Wattributes
* -Wdiv-by-zero
* -Wheader-guard