We inherited gzip compression API from spdylay codebase.  In spdylay,
the cost of having such API is almost free because spdylay requires
zlib for header compression.  nghttp2 no longer uses gzip to header
compression.  zlib dependency exists just for gzip compression API,
which is not an essential.  So we decided to move gzip code to under
src and remove zlib dependency from libnghttp2 itself.  As nghttp2
package, we depend on zlib to compile tools under src.

Also updates docs

Currently, nghttpd server only compresses files whose extensions are
one of .html, .js, .css and .txt.  nghttp advertises its support of
per-frame compression in SETTINGS frame.  To implement this feature,
we added 2 public API: nghttp2_session_get_remote_settings() and
nghttp2_gzip_inflate_finished().

This allows users of OS X 10.9 to run nghttpd (and friends) with
threading entirely disabled, to avoid crashes on startup related to
std::mutex.

libevent-server.c:552:43: error: missing field 'recv_callback' initializer [-Werror,-Wmissing-field-initializers]

libevent-client.c:355:43: error: missing field 'recv_callback' initializer [-Werror,-Wmissing-field-initializers]

libevent-server.c:691:8: error: C++ style comments are not allowed in ISO C90

nghttp2_session.c:1615:9: error: C++ style comments are not allowed in ISO C90 [-Werror]

nghttp2_stream.h:94:43: error: comma at end of enumerator list [-Werror=pedantic]

nghttp2_hd.h:115:30: error: comma at end of enumerator list [-Werror=pedantic]

nghttp2_int.h:50:33: error: comma at end of enumerator list [-Werror=pedantic]

referred to by the global variable 'long_options' upon returning to the caller.
This will be a dangling reference

Found by Clang Analyzer