Add AX_CHECK_COMPILE_FLAG (from Autotools packages)

It's fix the build with Clang and --enable-werror, don't support -WClobberred and display error about unknown warning option
error: unknown warning option '-Wclobbered'; did you mean '-Wconsumed'? [-Werror,-Wunknown-warning-option]
Add some news CFLAGS:
* -Wpragmas
* -Wunreachable-code
* -Wpedantic
* -Waddress
* -Wattributes
* -Wdiv-by-zero
* -Wheader-guard
Merge branch 'add_check_compile_flag' of https://github.com/alagoutte/nghttp2 into alagoutte-add_check_compile_flag
Fix compile error with clang -Wunreachable-code
Check max SETTINGS_HEADER_TABLE_SIZE in nghttp2_iv_check()

Hide NGHTTP2_MAX_HEADER_TABLE_SIZE from public API.  Now it is defined
as ((1u << 31) - 1) in nghttp2_frame.h, which is sufficiently big
enough.
Return NGHTTP2_ERR_BUFFER_ERROR from nghttp2_hd_{deflate,inflate}_hd

It is generally useful to know what is the cause of the error.  Since
we expose HPACK API, it is friendly to tell application the
insufficient buffer size is a culprit.
Add missing library error code to nghttp2_error()
Remove outdated comment
python: Return Handler object for pushed resource from BaseRequestHandler.push

Also update doc
doc: Add Python API doc
Add HPACK deflation API
src: Use gmtime_r instead of gmtime
src: Eliminate use of snprintf
src: Rewrite util::format_hex
src: Use C++ style comments for C++ source code
Update doc
Update doc
Update nghttp and nghttpd example output
nghttpx: Remove Connection: close header field from CONNECT response

It appears that some Android client does not like Connection: close in
200 CONNECT response.  Browsers fine with this header field.  It is
better to remove it.  Squid does not emit it too.
Update http-parser to 8d9e5db981b623fffc93657abacdc80270cbee58