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

This is simply programming error, but it is interesting that using
libstdc++ does not reveal this error.  With clang++-libc++, we got
std::system_error: mutex lock faild: Invalid argument.  This is
because we did not give a name to lock object, so it is immediately
destructed.  I think this will fix the reported crash on Mac OSX.

We may run into race condition if execve is called at the same time
when fcntl is called.  But we just does this for now to compile
nghttp2 applications under older kernel.

This also means that at least one stream whose dpri is
NGHTTP2_STREAM_DPRI_TOP exists, its siblings descendants have no
chance to send streams, even if their parent stream has
NGHTTP2_STREAM_DPRI_NODATA.

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