Minor configure updates to make things compile on OSX with openssl 1.0.0 already installed on the system

Conflicts:
	tests/main.c
	tests/spdylay_session_test.c
	tests/spdylay_session_test.h

We use following algorithm to select protocol:

 1. If server's list contains "spdy/2", this function selects
    "spdy/2" and returns 1. The following steps are not taken.

 2. If server's list contains "http/1.1", this function selects
    "http/1.1" and returns 0. The following step is not taken.

 3. This function selects "spdy/2" and returns -1. (So called
    non-overlap case).

This function invoked when request from remote peer is
received.  In other words, frame with FIN flag set is received.  In
HTTP, this means HTTP request, including request body, is fully
received.

If max concurrent streams limit is reached, SYN_STREAM frames are not sent
and backed off. If other type of frame is waiting in the tx queue, it is
sent first. We introduced another priority queue for this purpose.
In this change we did not add code to send RST_STREAM when SYN_STREAM is
received but max concurrent stream is reached.

If we return negative error code in that function, it means frame will not
unpacked and ends up to lose any information the frame contains.
The spec says it should send RST_STREAM with PROTOCOL_ERROR, so we need
at least stream ID.
Therefore, the check should be performed in
spdylay_session_on_syn_stream_received().

It now always select "spdy/2" as a next protocol regardless whether or not
the server advertises it. The NPN draft allows this.
Returning integer version number is not flexible because the selected protcol
is just a string.
The function now returns 0 if the server advertised spdy/2, or -1.

We use just single buffer to store name/value headers fields, instead of
allocating memory for each name/value strings.
It is now more than 2 times faster than old one.