When using IPv6 databases, IPv4 addresses are looked up as IPv4-mapped
IPv6 addresses.

Mostly based on a patch by Gregor Kališnik (ticket #250).

This was introduced for conditional compilation of the code that requires
the zlib library.

This includes "debug_connection", upstreams, "proxy_pass", etc.
(ticket #92)

To preserve compatibility, "listen" specified with a domain name
selects the first IPv4 address, if available.  If not available,
the first IPv6 address will be used (ticket #186).

On Mac OS X system toolchain by default prefers include files
from /usr/local/include, but libraries from /usr/lib.  This might result in
various problems, in particular the one outlined below.

If the PCRE library is installed into /usr/local/, this results in pcre.h
being used from /usr/local/include (with PCRE_CONFIG_JIT defined), but
libpcre from /usr/lib (as shipped with the OS, without pcre_free_study()
symbol).  As a result build fails as we use pcre_free_study() function
if we try to compile with PCRE JIT support.

Obvious workaround is to the root cause is to ask compiler to prefer
library from /usr/local/lib via ./configure --with-ld-opt="-L/usr/local/lib".
On the other hand, in any case it would be good to check if the function
we are going to use is available, hence the change.

See thread here for details:
http://mailman.nginx.org/pipermail/nginx-devel/2012-December/003074.html

Prodded by Piotr Sikora.

Very basic version without any OCSP responder query code, assuming valid
DER-encoded OCSP response is present in a ssl_stapling_file configured.

Such file might be produced with openssl like this:

openssl ocsp -issuer root.crt -cert domain.crt -respout domain.staple \
             -url http://ocsp.example.com